Migrations: Add some "kommands"

This is part of #342. It adds the following commands.
* k ef migration add [name] -context &lt;context&gt;
* k ef migration apply [migration] -context &lt;context&gt;

Verification: Manual bashing. Tests to follow.

![image](https://cloud.githubusercontent.com/assets/1226749/4261593/edde2f90-3b75-11e4-8cef-e368cfab0ee7.png)

Should these say "DbContext" for clarity?