Move lexicon out of Avocado core

Fix #183.

This follows the same steps django-objectset recently underwent when it was removed from Avocado proper. There is no longer a hard dependency on Lexicon here. Note, I renamed the lexicon subcommand file to _lexicon to avoid issues with imports within the subcommand files. If the file was named lexicon, then the subcommand would be imported instead of the lexicon library.

Signed-off-by: Don Naegely <naegelyd@gmail.com>