This commit adds a problem matcher for Checkstyle to allow
automatic GitHub annotations.
When run through Gradle, an example line is:
```
[ant:checkstyle] [ERROR] /root/Bound.java:7:2: 'import' has incorrect indentation level 1, expected level should be 0. [Indentation]
```
Alternatively, when run standalone, an example output is:
```
[WARN] /root/test.java:2:1: The name of the outer type and the file do not match. [OuterTypeFilename]
```
The regular expression matches both cases.
In the latter example, `WARN`, rather than `WARNING` is output.
This is resolved by using a default severity of `warning`, which
will be overridden in the case that the severity is `ERROR`.
