Configure command

This pull requests is work towards making Humboldt usable outside of Burt. Hard-coded Burt specific configuration (and defaults) for EMR jobs, namely EC2 key name, job bucket and data bucket have been removed.

A command, `humboldt configure`, which creates a project configuration in a file called `.humboldt.yml` was added to quickly set up the necessary EMR configuration.

Would it be a good idea to warn (or even error) the user when no `.humboldt.yml` exists during `humboldt run-emr`? This would make it easier to understand what's going on for developers of existing projects upgrading past this version of Humboldt - as well as newly created projects.

`humboldt configure` is idempotent, i.e. hitting enter all the way through preserves the configuration file as it was. If the user enters a default value, it will not be written to the configuration file.