Add conda env remove command

This is largely copied from conda.cli.main_remove.  There's probably
room for sharing code between these two code paths, but it's easier to
bring over here for now.
Err -- fix copy-n-paste error and add a note
Add another TODO
Ensure root env is checked first
Tweak error message and provide next step
Merge pull request #4 from conda/feature/remove-cmd

Add conda env remove command