Add shebang to update scripts and fix Xcode references
Weak-link against libssh2.

This allows access to the `ssh` transport if you have libssh2 installed (eg. using `brew`, so I'm only looking it `/usr/local/lib`). The libgit2 cmake script will detect it automatically, but since we're building static, it's our responsibility to have it.
Merge pull request #200 from tiennou/script-fix

Add shebang to update scripts and fix Xcode references
Revert "Weak-link against libssh2."

This reverts commit 06d171dc43b8aae2967c0d1e8767da79acd88309.

@tiennou This is failing on my machine and the build server, due to libssh2 not being found. It sounds like the weak option isn't working correctly?
Implement <NSCopying> for GTOID.
Add init method to initialize GTOID with a SHA C String and use this in the initWithSHA: method.
Add convenience methods to create GTOID objects.
Fix style issues.
Add unit test for -initWithSHACString:
Fix more style issues.
Add documentation and fix style issues for the convenience initializers.
GTEnumerator; don't convert an OID to a SHA just to get an OID

Saves ~15% runtime when walking a large tree (linux kernel)

Improvement for libgit2/objective-git#187
Merge pull request #207 from rowanj/walkPerformance

GTEnumerator; don't convert an OID to a SHA just to get an OID
Add initializers that can return a NSError instance on error.
Fix formatting of documentation comments.
Better error message for invalid SHA string.
Test that an error is returned when GTOID is initialized with an invalid SHA string.
Merge pull request #206 from 5sw/gtoid

GTOID improvements