Fixes the bug where pants doesn't recognize build targets on top-level directories.

The previous argument parser naively simply checked whether a command-line argument contained a slash (/) or a colon (:). This is a reasonable assumption most of the time, but in the case where a BUILD file is located only one directory deep, (eg, buildroot/folder/BUILD), this makes it impossible to run pants on the target simply by calling (eg) ./pants goal bundle folder.

See github issue for example: https://github.com/pantsbuild/pants/pull/159

This fixes it by actually checking to see if a phase with goals is defined by the command-line argument. It still checks before that whether the argument contains a slash or a colon, because that if an argument has those characters it can't mean a goal. But an argument not having those characters doesn't mean it is a goal.

In a completely ambiguous situation (eg, given a command argument 'bundle', and there is a valid BUILD file at bundle/BUILD), we err on the side of the goal, and print out a warning. This makes sense, because it allows the user to disambiguate the situation using the '--' syntax.

Testing done:
Added unit tests to pants_test/commands/test_goal.py; everything passes.

Bugs Fixed 218

Reviewed at https://rbcommons.com/s/twitter/r/551/