Overcame yet another backwards-incompatible change to v8.

This time, String::New no longer exists:

    https://github.com/v8/v8/commit/e25722518bb03000ec288393c1600190adfa5166
Fixed ExecuteJs, another victim of a backwards-incompatible v8 change:

    https://github.com/v8/v8/commit/2c2e0302deb8bd658a9ab8769c4229afab828e26
Fixed missing Isolate parameters, due to yet another v8 change:

    https://github.com/v8/v8/commit/7041b78f2fc07a9f8879e52bb4afccb5dc7af86b
Updated uses of MakeWeak, deleted in a backwards-incompatible v8 change:

    https://github.com/v8/v8/commit/b63073e6bb4839c364763fccb55cb5da5ae0742d
Fixed yet more v8 incompatible API change breakages.
Added an include path for homebrew's version of libxml2.

Apparently this is no longer in /usr/include like it used to be on OS X.
Fixed -lprotobuf link errors on OS X 10.9.3.
Fixed some test build errors.
Fixed bugs in how SetWeak was used.
Fixed integration tests.
Made everything build and pass again with v8 3.25.30.

Fuck.