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
respect `pbyte_offset` with cstubs

fixes #165
(Failing) test for pointers with offsets in Cstubs.
Merge branch 'pqwy-fix-raw_ptr'
Remove the pbyte_offset field.
Remove the ~offset argument to Memory.build.
Remove the offset arguments from ctypes_memcpy.
Remove the offset argument to ctypes_read.
Remove offset arguments from ctypes_read_pointer and ctypes_write_pointer.
Remove the offset argument from ctypes_write.
Remove the offset argument from Ctypes_bigarray.view and supporting stubs.
Remove offset argument from Memory_stubs.string_of_array.
Remove offset argument to ctypes_string_of_cstring.
Merge pull request #168 from yallop/remove-offsets

Remove offsets
Update comment for OCaml 4.02.0.
Move CArray implementation to a separate file.