Updated Dockerfile

Key changes in Dockerfile:

- Updated to Trusty Tahr debs
- `apt-get install` commands separated out as different `RUN`s
- Builds with clang
- No longer clones the repository, but instead includes local checkout via ADD.  This should speed things up and make it easier to test in forks and branches.

*Note* this includes the changes from numenta/nupic#817, which fixes numenta/nupic/issues/634.  See https://index.docker.io/u/oxtopus/nupic-ubuntu/build_id/3886/code/bvcspecdwnkfwcp9jae7pj9/ for trusted build details.