Add verify-hosting-dependancies task

The `verify-hosting-dependancies` task checks that everything required to launch the dev instance of WordPress (eg, MySQL) has been installed, and if not, fails the `grunt run` task with a helpful error message.

eg:

```
vagrant@precise64:~$ grunt run
...snip...

Running "compile-buildpack" task
-----> compile params: BUILDPACK_DIR: /home/vagrant/buildpack BUILD_DIR: 
...snip...
-----> Done with compile

Running "verify-hosting-dependancies" task
>> mysql must be installed
Warning: Task "verify-hosting-dependancies" failed. Use --force to continue.

Aborted due to warnings.
```