Support for a load of features

[hayd]

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

• [ ] cache the builds better
• [x] 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?)
• [x] Rather than always do local and global test env, support substitutions
• [x] support more substitutions
• [ ] lots more...

~~Possible to use test suite for tox?~~

~~I will probably never do these.~~