Log messages, in a scalac-like way, to sbt, not println.

This sends messages to sbt's `Logger` rather than using `println`.  Each level of scalastyle warning is distributed to the like-named level of the sbt logger.  When the `w` option is passed to `scalastyle`, `warning`-level messages are printed at `error`-level instead.

I favor the `scalac`-like location format because Emacs `compilation-shell-minor-mode` knows how to jump to locations formatted like that, but I think it is nicer for everyone regardless.

Imagine this with colorful level boxes, which is how it prints on a terminal:

```
> scalastyle
[warn] /home/sirian/ide/sbt-project/mainline/src/main/scala/com/nocandysw/EvHelpers.scala:25: File line length exceeds 80 characters
[warn] /home/sirian/ide/sbt-project/mainline/src/main/scala/com/nocandysw/EvHelpers.scala:11:58: Usage of ??? operator
[info] Processed 4 file(s)
[info] Found 0 errors
[info] Found 2 warnings
[info] Found 0 infos
[info] Finished in 11 ms
[success] created: sbt.SettingKey$$anon$4@463bffda
[success] Total time: 1 s, completed Feb 11, 2014 11:35:40 PM
> scalastyle w
[error] /home/sirian/ide/sbt-project/mainline/src/main/scala/com/nocandysw/EvHelpers.scala:25: File line length exceeds 80 characters
[error] /home/sirian/ide/sbt-project/mainline/src/main/scala/com/nocandysw/EvHelpers.scala:11:58: Usage of ??? operator
[info] Processed 4 file(s)
[info] Found 0 errors
[info] Found 2 warnings
[info] Found 0 infos
[info] Finished in 2 ms
[success] created: sbt.SettingKey$$anon$4@463bffda
[trace] Stack trace suppressed: run last *:scalastyle for the full output.
[error] (*:scalastyle) exists warning
[error] Total time: 0 s, completed Feb 11, 2014 11:35:43 PM
```