Async scanning infrastructure

Adds an infrastructure for doing async scans to the VIP Scanner.

In this PR, the active theme will be scanned via cron every 15 minutes. The number of errors, warnings, and issues are shown in an admin bar node.

While doing the cron, the whole review is done against every file, however the new `AsyncDirectoryScanner` class is also designed to support partial scans (aka running checks against a subset of files to reduce scan time).

Scan results are stored as a CPT with the path to the files being scanned as the post name, the results in a post meta, a review taxonomy, and a post meta with counts for each different type of issue (blocker, warning, note). When a manual scan is performed it overrides the last async scan in the cache.

![vip-scanner-async](https://cloud.githubusercontent.com/assets/1120274/2712569/e994ed18-c4db-11e3-9884-fda1c61779b4.png)


See #73