Add ability to guess app name from dirname

This is *in addition* to guessing app name from the `tsuru` git remote.

The `tsuru` git remote is tried first and if that doesn't work, then we
take the name of the current directory and look to see if there is an
app with that name. If there is, we use that; else error out.

The main reason for adding this is that I am thinking of adding another
command that would add the git remote for the user.
Merge pull request #976 from msabramo/guess_name_of_app_using_dirname

Add ability to guess app name from dirname