ENH: enhance set_option syntax
closes #3667.

who says my schemes are evil :)
well...maybe a little
 k removed it sorry for the unnecessary stuff
 @y-p this ok?
 rebased
 a7d404ca9dc1a2869b4c49bb46e5c5385452955f is the latest
 the original reason if you're interested was because [`ast.literal_eval`](http://docs.python.org/2/library/ast.html#ast.literal_eval) was exactly what was needed: a safe eval that only evaluates basic python objects
 The commit subject for a7d404c says it's concerned with the test suite, which Is why review is confusing.
with squashing, look fine to me.
 oh sorry ok i will be more careful with my commit messages
 squashed
 @jreback this has been squashed and is ready to go.
