Constrained B-spline

Last part of the #1 PR: constrained B-splines. I cleaned up a bit, moved test checks to Boost, and fixed an error in the coupled constraint. Example on this screenshot (taken from the test):

![constrained_bspline](https://cloud.githubusercontent.com/assets/2742231/2690645/921678d0-c356-11e3-9ff9-607578ef7654.png)

The following constraints are set:
* Fixed constraints: P(0.) = P(1.) = 0, P(0.5) = 1
* Coupled constraint: P'(0.25) = P'(0.75)