Added support for sets and frozensets

[svisser]

Hi there, this is a proposed fix for #292.

The docstring was done slightly differently than the others as the behaviour differs in the Python shell between Python 2.7 and Python 3.1+ (set([42]) vs {42}):

>>> set([42])
set([42])

>>> set([42])
{42}

The raised Invalid exceptions aren't used to avoid the problem that sequences currently have:

Schema([int,str])([3.4])

results in voluptuous.error.MultipleInvalid: expected str @ data[0] (the error message can be considered true but also incomplete as int is also allowed).

I'm not sure how to solve this - this could perhaps be refined in a future pull request?

[coveralls]

Coverage increased (+0.1%) to 95.366% when pulling 4d742e4 on svisser:feature/support_sets into 72411cd on alecthomas:master.

[tusharmakkar08]

Hey @svisser

Thanks for the PR. Can you add the inclusion of sets and frozensets in README.md as well?

Thanks

[alecthomas]

Can this be type_name = type_.__name__? That will also handle user-defined subclasses of set, etc.

[svisser]

Thanks, I have updated the commit with the requested changes. If there are any more just let me know.