Teach (iter)solve to take generic iterables of iterables as clauses argument

Previously, you could call `solve` (and `itersolve` implied by `solve` from here on out) only with a list of lists of ints. Alternatively you could call `solve` with an _iterator_ of lists of ints. This pull request adds the ability to call `solve` with a generic iterable of iterables of ints, for example a tuple of generators of ints.

Tests for both `solve` and `itersolve` pass on Pythons 3.3.2 and 2.7.5 (using the patch from #5) on Mac OS with compiler Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn) targeting x86_64-apple-darwin12.5.0. I don't have access to Windows or Linux.

This addresses issue #3.