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.

This looks like this...

![image](https://cloud.githubusercontent.com/assets/511795/3484043/46e8016c-039d-11e4-8b4d-6716a81c9a53.png)