Taught klee_warning(), klee_error() etc... to emit coloured text output.

Also use bold green text when KLEE finishes. This is done by taking
advantage of llvm::raw_ostream's nice API for controlling the console
text colour.
Merge pull request #134 from delcypher/coloured_error_messages

Taught klee_warning(), klee_error() etc... to emit coloured text output.