Use gyp instead of cmake to build libgit2.

It's too cumbersome to setup all the building environments on Windows
with the default cmake, using gyp would reduce to building requirements
to minimum.
Fix building on Windows.
Remove unneeded string copy.
Update node-cpplint to v0.1.5.
Update jasmine-focused to v0.15.0.
Supress compiler warnings on Windows.
VLA is not available in MSVC.
Fix specs that has wrong path delimiter on Windows.
Fix repo.relativize on Windows.
Skip the symbol link test on Windows.
Fix git.open on Windows.
Merge pull request #7 from atom/gyp

Port to Windows