Support for a load of features

[hayd]

Feature list is basically here http://tox.readthedocs.org/en/latest/config.html

• cache the builds better
• parse_args, and have {options} and friends work (so can do ctox --pdb !)
• changedir (should default to .tox and do the installations and tests there?)
• Rather than always do local and global test env, support substitutions
• support more substitutions
• lots more...

Possible to use test suite for tox?

I will probably never do these.