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

This change enhances error highlighting in nrepl.

Key Features:
* Highlights both errors and warnings (warnings are typically seen when ``*warn-on-reflection*`` is set)
* Highlights only the containing sexp rather than whole line, which allows a warning and error on the same line. If the compiler started emitting multiple errors, each would show separately
* Displays compilation error message in minibuf on ``M-x next-error`` and mouse hover.

In emacs < 24.3 it looks like this:
![Screenshot - 150313 - 18:06:43](https://f.cloud.github.com/assets/373335/264819/d8b5ef64-8d9c-11e2-9c75-cc6eb28fd528.png)

In emacs 24.3 and later, you can use the wave underline property. Add this:
```lisp
(set-face-attribute 'nrepl-error-highlight-face nil :inherit nil :underline '(:color "red4" :style wave))
(set-face-attribute 'nrepl-warning-highlight-face nil :inherit nil :underline '(:color "gold4" :style wave))
```
to ``init.el`` (or equivalent) and it looks like this:
![Screenshot - 150313 - 18:29:39](https://f.cloud.github.com/assets/373335/264853/545972fc-8d9e-11e2-9b4d-653323b25146.png)