Implement Context::GetIsolate()

BUG=
TEST=test-api.cc (IsolateOfContext)

Review URL: https://codereview.chromium.org/11968011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13397 ce2b1a6d-e550-0410-aec6-3dcde31c8c00