Support excluding of warnings via regex pattern

Dear @bolinfest, I am hoping that you may consider this pull request.

I believe this will allow a close  to [Issue 1: Make it possible to suppress warnings from individual JS file](https://code.google.com/p/plovr/issues/detail?id=1).

Example usage
```c
  # plovr.cfg
  {
    "warning-exclude-paths": [
    "^/closure/goog/testing/stacktrace.js",
    "^/closure/goog/iter/iter.js",
    "^/closure/goog/labs/promise/promise.js",
    "^/closure/goog/labs/promise/thenable.js",
    # transitive dependencies seem to resolved more fully...
    ".*?/closure/goog/mochikit/async/deferred.js"
    ],
  }
```
In the interests of other warning suppression, this branch also updates the Config.getJsContentType method to return `application/javascript` instead of `text/javascript` since the latter seems to upset HtmlUnit:

```syslog
WARNING: Obsolete content type encountered: 'text/javascript'
```

Many thanks for considering this pull request, and for plovr itself.

Sincerely,
Rick Power