Tidy up some naming choices

- pkg -> package for consistency
- class -> package in all cases not referring to Test::Kit2 itself
Improve external facing documentation (POD)

Add a DESCRIPTION section.

Add comments to the code example in the SYNOPSIS.
Include the target in the fake package name

This should keep things working even if a project has two different
Test::Kit modules which both use Test::More, for example.

I'm too lazy to add a test that actually checks this works.
Setting the TB file handle I/O layers works

Unfortunately this test has to be checked by eye because I can't for the
life of me figure out how to test it without compromising what we're
actually trying to test! It works though ;-)