* The inner mouth corners are not mapped since I think they don't realy exist in the 3D model.
* Some points are not defined on the reference and only present in the 3448 model and above.
* A few of the selected 3D vertices could be improved a bit, I have some notes on it, if someone is interested, contact me.

If not given, the mean will be used, which is the right thing in that case.

This is used for example when we fit blendshapes and want to fit the PCA shape given current blendshape parameters.

See GitHub issue #4 for details.

This runs 10-30% faster (and is potentially more stable).

Based on the linear landmark fitting.
The regularisation is set conservatively, in the experiments so far even a zero value yielded good results.

The file contains blendshapes for these 6 facial expressions:
angry, disgust, fear, happy, sad, surprised.

These are reduced in number of vertices to match the low-resolution (3448 vertices) shape model.
They should be considered beta-quality.

It calls the two existing functions, so it requires less code on the application side.

Also added a few notes to the documentation

- Updated ibug2did.txt with a description of the contour indices
- Added a file describing the left and right model contours (the face outline) on the reference model
- Caveat: The implementation is a bit poorly documented for now

* texture is now correctly a boost::optional

Notes:
* Removed the runtime-check in get_mean_at_point()
* I couldn't find out whether ::rowRange has a bounds check in debug builds. Therefore, added an assert.

Conflicts:
	README.md

Added comments to texture extraction

mainly on the texture interpolation methods

v0.6.0: OpenGL-like rendering, texture rendering, texture extraction view-angle computation