Pull emp CLI into cmd/emp

[ejholmes]

Depends on #710.

One of the only reasons emp was in a separate repo previously was because we needed to rewrite import paths with godep save -r ./... to make it "go gettable" with Go 1.5+ and the vendor directory, this is no longer necessary.

This has multiple benefits:

1. Releasing new features is more "atomic" since we can release a daemon and client that are always compatible with each other.
2. It was common for CLI tests to fail before because something wasn't merged into remind101/emp. This won't be an issue anymore.

TODO

1. Update docs about installing emp.
2. Add a deprecated notice on remind101/emp.