Also renamed the app that creates the bindings to generate-python-bindings.cpp.

- Added a flag GENERATE_PYTHON_BINDINGS to CMake which enables the use of pybind11
- `create-python-bindings` generates bindings for PcaModel
- pybind11 is added as a submodule; needs to be cloned with `git submodule update --init`
- Increased cmake_minimum_required to 2.8.12 for pybind11

Changed all affected functions and classes accordingly: Mainly the renderer, affine-renderer, and affine texture extraction.

This results in a ~35% speed-up of rendering meshes.

It requires some not-so-beautiful conversions in render_affine and the current texture extraction, but they will be superseded by the new renderer soon anyway.

Fits camera, shape, blendshapes and colour coefficients with a cost function containing a landmark constraint, priors, and minimising the error between the input image's RGB values and the model estimate (rendered at each vertex).

Note: The image cost function cannot be used with the model on GitHub, as it doesn't include the colour PCA.

Not sure what's the best place to put them, let's put them here for now.

* Fits the pose (camera) to landmarks using the newly added Ceres cost functions
* Added find_package(Ceres) to CMakeLists.txt

He updated it quite a few months ago and sent the file to me - now finally committed.

* The changes were necessary to fix issues with Ceres's Jet type in glm vector and matrix types
* Added glm-git-modified-readme.txt to explain these changes

* Added const and non-const overloads. The non-const is needed because of the RNG in PcaModel.
* Renamed a few variables for consistency

This is needed by the json-to-cereal-binary app to read the JSON generated from the Matlab BFM converter script.

Otherwise, it looked like JSONlab will not write the output file, probably because it's >1GB. See GitHub issue #29.

Renamed fit_shape_model to fit_shape, since it's fitting the shape in general (including blendshapes), and not only the PCA model.

Added a BUILD_UTILS CMake option, defaulting to OFF.

- Added expression fitting
- Added Surrey Face Model section
- Slightly restructured build and sample section
- Replaced ICIP with the VISAPP publication
- Replaced license badge with static one

Related to last commit: Added serialisation methods for boost::optional. Copied the file from https://github.com/USCiLab/cereal/pull/122, hopefully that pull-request will be accepted.
This commit adds the file to CMakeLists.

I don't think we need two representations for it, one struct with a type is easier. At the moment, we only use an orthographic camera anyway.

- As before, it should contain all the parameters necessary to render an image using OpenGL, and conform to the OpenGL conventions
- Updated all calling sites
- Added serialisation
- Added a convenience method to save the parameters to a json file

Probably needed because OpenCV needs it, and we depend on OpenCV.