AD2: Add POD Documentation
AD2: Convert from AD2USB -> AD2

All references to AD2USB has been replaced with AD2.

This means all configuration settings and user code settings need to be updated as well.

The change was necessary because this module will work with at leaast AD2Serial, AD2USB, AD2Pi, and likely many more AD2 style products that will be released in the future.
AD2: Rename AD2USB.pm -> AD2.pm
AD2: Add MHT definition for AD2_INTERFACE