Merge pull request #64 from AlexanderFabisch/master

Remove unused import.
Add CG test case.
Simplify CUDA helper function.
Remove loop and throw OpenANNException.
Fix bug in logger.
Allow step-wise CG optimization.
Merge functions in LMA.
Refactor CG.
Merge pull request #65 from AlexanderFabisch/refactor-optimizers

Refactor optimizers