Add links to RWO and the Mirage blog in the README.
Change errno checking implementation in the foreign backend.

Whether to check errno is now recorded in the callspec, and
implemented as a branch in ctypes_call.
Support for releasing the runtime lock during calls to C.
Tests for bindings that release the runtime lock.
Merge pull request #164 from yallop/dynamic-call-contexts

Support for releasing the runtime lock