Fix #211 - implement jump to error from stacktrace

Now when someone gets a compilation error after `C-c C-k` the top error
of the stacktrace is clickable and it will take the user to the
offending line in the source code. This doesn't work when there is
no source file (say after eval last sexp) for obvious reasons. The functionality
relies on `compilation-minor-mode` being enabled in the error buffer (which is done
automatically after it's created).
Highlight compilation error in the source buffer

The compilation error's line number is extracted from the stacktrace
buffer and highlighted with an overlay in the related source buffer.
The error overlay is cleared upon recompilation. There is some race condition
between the moment in which we start to parse the stacktrace buffer and the moment
the stacktrace has been output to it, therefore the 0.3s sleep to suppress it.
Mention race condition in a comment
Don't underline the blank beginning of the error line
Use font-lock-warning-face as a basis for nrepl-error-highlight-face.
Remove debug message.
Fix function indentation level.
Merge branch 'compilation-mode' of https://github.com/bbatsov/nrepl.el into bbatsov-compilation-mode