Improved phrasing of notice that validation step is skipped.

Conflicts:
	pymavlink/generator/mavgen.py
generate.py is now Python 3 compatible. XML validation doesn't work in Python3.

Conflicts:
	pymavlink/generator/mavgen.py
Merge pull request #104 from Susurrus/new_fix_gen

Fix generator GUI for Python3 where validation libraries are not available