Basic string generation from a Regexp

This will turn any regexp containing characters, \w, \d, and \s into a string
that matches the regexp.
Recognized escaped characters
Pick characters from sets

This will generate parts of the string from a regexp set of characters,
eg /[ab\w]/
Handle question marks on atoms

When we find a question mark, we skip the current token with probability 1/2.
Handle stars on atoms

When we find a star, we either remove it and throw away the token or print the
token and leave both it and the star in place to run again.
Handle pluses on atoms

When we find a plus, we either consume it and print the token or leave both on
to run again.
Handle ?, + and * with special tokens

This lets you generate a varying number of escaped character sets like `\w`.
Currently, `?` will always generate exactly 1 of its target—never 0—because that
let me get `+` and `*` working in a simple way. It's technically correct, and
letting it generate 0 made it harder to work with '+' and '*'.
Add regexp files to gemspec
Update for Ruby 1.9.3

This must be the oldest branch I've ever had.
Move Faker::Regexp -> Faker::String

It could be confusing that this was called `Regexp` and generating a `String`
when everything else generates what it's named.
Test using multipliers on sets

This happily just fell out of the implementation. Tests make it official.