Add .gitignore

Hi chokkan,

thanks for your amazing library! I'm using liblbfgs as a git submodule in my own MRF-learning project. Everytime I compile liblbfgs, the various files created by the compilation process show up as uncommitted changes in the liblbfgs tree and ergo in my main project. 

This gitignore file, which is a combination of the gitignore boilerplates at github/gitignore for C, autotools, vim and some liblbfgs-specific stuff, should make this more comfortable.

Best,
Stefan