test: proposal to add support for travis-ci

This adds support for [travis-ci](https://travis-ci.org) for Boost.Build.  It makes a few minor fixes in the test infrastructure as well as adds a Makefile to control the process.  This could use a lot more work to support multiple toolsets and platforms, but I thought I would propose it and see if it seems interesting.

This is currently building from a [fork](https://github.com/tee3/build) of Boost.Build.  To enable it for the main repository, you would just have to add a key to the main repository and turn it on.