Fix internal memory management on the library

Fix internal memory management on the library

String mememory is now managed in a much more sane manner.

Fixes include:

	- git_person email and name is no longer limited to 64 characters
	- git_tree_entry filename is no longer limited to 255 characters
	- raw objects are properly opened & closed the minimum amount of
	times required for parsing
	- unit tests no longer leak
	- removed 5 other misc memory leaks as reported by Valgrind
	- tree writeback no longer segfaults on rare ocassions

The git_person struct is no longer public. It is now managed by the
library, and getter methods are in place to access its internal
attributes.

Signed-off-by: Vicent Marti <tanoku@gmail.com>