Implemented parameters for Diff.find_similar()

Diff.find_similar now supports the parameters that are exposed by
libgit2's git_diff_find_similar.

Parameters supported are rename_threshold, copy_threshold,
rename_from_rewrite_threshold, break_rewrite_threshold and rename_limit
Parsed parameters directly passed into options object
Merge remote-tracking branch 'ArnoVanLumig/master'