Support for better flashsms.

I bumped Webmock, split up the specs so they're easier to read and added better support for flashsms. I think it's better to coerce the flash value, which preferably is a boolean, in the library itself instead of the user needing to do it.

Of course, ```:flashsms 'yes'```, still works.

Maybe if the coercion and checking of parameters continue to grow, it should be extracted out of the ```send``` method. What do you think?