Merge pull request #9 from bchretien/master

Fix a bug with SolverError
Add Travis support.
Update .travis.yaml to add Ipopt dependencies.
Update .travis.yaml to add Ipopt dependencies.
Add build status.
Fix typo in README.md
Allow clang to fail in Travis.
Start sparse matrix support implementation.
Finish implementing sparse matrices support.
Prepare test suite for sparse matrices support.
Add sparse matrix test.
Install Eigen-3.1.2 in Travis build script.
Force gcc use when compiling Ipopt in Travis.
Fix clang warnings.
Disable new_x support for now.
Reorganize test suite.
Synchronize.