* Implemented git_diff_index_to_tree
* Reworked git_diff_options structure to handle more options
* Made most of the options in git_diff_options actually work
* Reorganized code a bit to remove some redundancy
* Added option parsing to examples/diff.c to test most options

This fixes several bugs, updates tests and docs, eliminates the
FILE* assumption in favor of printing callbacks for the diff patch
formatter helpers, and adds a "diff" example function that can
perform a diff from the command line.

Reverted signature of git_signature_new.
Removed error check wrappers (voted down). Made Makefile
work out of the box on Linux and Solaris when standard
cmake build instructions for the library are followed.