Separate MPL and MPL.Core

The pull request moves some core components to a separate sub-library Boost.MPL.Core. In particular, the following components are moved:

- intergal constant wrappers and void_
- arithmetic, logical, comparison and bitwise operations
- diagnostic tools like print and BOOST_MPL_ASSERT
- if_, eval_if, sizeof_, has_xxx, identity

To remove the dependency of the core components on the TypeTraits library the workaround for Borland Compiler 5.9 has been removed from has_xxx.hpp. As a result MPL.Core headers only depend on Config, Preprocessor and Core.

The tests for the moved components were also moved to the sub-library. Also created a readme file.

This pull request depends on fixing the Jamroot file (see https://github.com/boostorg/boost/pull/35) so that 'b2 headers' work correctly for the core headers.