Enhanced error highlighting. errors in red, warnings in yellow.

This adds a new face for warnings and tweaks the highlighting code to use it. Warnings are typically Reflection warnings when compiling with ``*warn-on-reflection*``
 set.

A further enhancement would be to specify regexps to match errors / warnings. There is ``nrepl-compilation-regexps`` that might be extendable for that.

I have a further improvement on my master branch which uses emacs 24.3's 'wave' underline to highlight the errors with a wavy line, but preserving the existing font-lock highlighting, that looks something like this:

![Screen Shot 2013-03-14 at 12 38 05](https://f.cloud.github.com/assets/373335/258664/1e4ba3d8-8ca4-11e2-9d54-aabdd85c7e27.png)