Added documentation and updated training for LLVM

The training tools have been updated so that they also cover the LLVM
workflow. The scripts have also been cleaned up a little so they should
be easier to follow.

Preliminary documentation has been written to cover the usage of the
training tools.

    * doc/mageec.texi: Documentation written to cover usage and
    workflow.
    * training/clang.py: Added. This script is used to drive the
    training process for LLVM.
    * training/evaltool.py: Updated to work with LLVM
    * training/matrixgen: Added. R script used to generate a matrix of
    flags or passes to train the database. Renamed from 'tablegen'.
    * training/test-exec.sh: Renamed from test-pt2.sh. Updated to work
    with LLVM.
    * training/test.py: Updated to work with LLVM.
Updated evaltool.py to read in execute.log

Previously evaltool.py would read in from beebs.log which was a
processed version of execute.log with the calibration test benchmarks
removed, however this should now be handled by the script.