Allow 'use X tests => 4' syntax

This means making the target package a Test::Buider::Module instead of
just a plain Exporter.
More improvements to the Synopsis
Fix typos!
Support "include 'Mod1', 'Mod2';" as a shorthand
Capitalize exception string for consistency
Document all the exceptions Test::Kit2 can throw
Test that custom subroutine exports work

Allow the user of Test::Kit2 to specify custom subroutines within the
kit-using module and export them fairly easily.