Merge pull request #16 from florent-lamiraux/master

Fix uninitialized value in SplineLength class.
tests/vector-interpolation.cc: fix previously dead loop
include/roboptim/trajectory/vector-interpolation.hxx: implement derivative before/after singular points
Add tests for spline length (basic one, to be extended)
tests/cubic-b-spline.cc: use clone function
tests/vector-interpolation.cc: use clone function
Update README.md (fix typo).

:lipstick: [skip ci]
Fix warnings, enable -Werror again
Fix some -Werror errors.
src/CMakeLists.txt: install library in LIBDIR
Fix a few warning by using size_type instead of unsigned.