Update to libgit2 0.17.0.
Merge pull request #62 from arthurschreiber/upgrade_libgit2

Update to libgit2 0.17.0.
pull in latest libgit2
add new exception classes
whitespace
stub out newish error handling
only deal with errors if there is one
not sure how in the hell...
lets also make sure an error was returend
ensure giterr_clear is called before raising
raise specific exceptions too
ensure specific errors are raised
update libgit2
simplify error checking logic
don't inline rugged_exception_raise
unused varables in rugged_exception_check
A more Neckbeard Friendly way to do a switch
Externs not needed
missing definition of rb_eRuggedError
OsError was renamed to OSError