Make Moose::Exception stringification eliminate internal Moose & CMOP frames at top of trace by default

If the MOOSE_FULL_EXCEPTION env var is true these frames are included.

This doesn't affect the frames that are actually captured, just the stringification.
Doc updates for exception class & manual
Fix some tests of exceptions to check for the actual error message we care about

The test was checking for one word that appeared in the trace rather than the
specific error message we expect to be thrown.
Merge pull request #82 from moose/topic/less-moose-in-errors

By default, exceptions now eliminate most of the Moose internals from stack traces