Add Plugin Agent installation, Berks-ify cookbook, add tests

This resolves #21.

There is, however, a decent bit going on here, so: 

* Adds Plugin Agent installation, primarily using code from [newrelic-ng](https://github.com/flinc-chef/newrelic-ng)
* Sets up this cookbook to be used with Berkshelf
* Adds tests using Test Kitchen and MiniTest, as per the example of newrelic-ng. Current tests are:
    * Repository
    * Server Monitor
    * PHP Agent
    * Plugin Agent (largely unchanged from newrelic-ng)

The README has been spruced up a bit, and the version number bumped to 0.6.0 to reflect the magnitude of these changes.

I did run into some difficulty getting the node to converge without a valid New Relic key; the Plugin Agent appears to install without trouble without a valid key, but the Server Monitor and PHP Agent do not, which makes running the tests as-is impossible. Temporarily inserting your own key works well, but there's a danger of committing that key. Suggestions on how to get around this difficulty are welcome.