Testem runner integration for anvil.js

This adds testem integration to anvil. Testem is my favorite testing tool and getting it into anvil is a big win.

Testem is activated by adding a --testem argument to the anvil command line:

anvil --testem

It's recommended that you run testem in conjunction with Anvil's CI mode:

anvil --ci --testem

This will cause anvil to rebuild your solution on any file change and then re-trigger the test suites to run in testem.

The configuration section for this plugin matches testem's configuration options:

"anvil.testem": {
    "port": which port to run testem at - default is 7357
    "launch": list of launchers to use ( Chrome, Firefox, PhantomJS, ...)
    "test_page": the page to use to run tests ( example: "spec/index.html" )
    "timeout": timeout for a browser
    "framework": test framework to use
    "src_files": files or patterns - unlikely you'd ever use this w/ anvil