Use GTFileModeBlob for blobs in GTTreeBuilderSpec

Fixes #191.
Add GTSubmodule.URLString
Merge pull request #197 from libgit2/submodule-url

Add GTSubmodule.URLString
Merge pull request #196 from libgit2/fix-file-mode-test

Use GTFileModeBlob for blobs in GTTreeBuilderSpec
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?