Use less unwieldy package and module names

[sajith]

There are two parts to this:

1. The package name "fabrictestbed-extensions" (and the PyPI project name) do not reflect accurately what the package does: this is is a collection of higher-level abstractions to work with FABRIC testbed, and we often call it "FABLib". Since the name fablib is already taken in PyPI, we need to think about a name: "fabrictestbed-fablib" perhaps?

2. The current way to import FABLib is perhaps a little too unwieldy:

from fabrictestbed_extensions.fablib.fablib import FablibManager as fablib_manager

We should look into a more easier to use module arrangement, while maintaining backward compatibility.