Add public class method ssh_public_key_bits

I've found it useful to pull out public SSH Key bits.  To that end, I broke out the logic of `::valid_ssh_public_key?` into a private method and use that data for both validation and `::ssh_public_key_bits`.  In both cases, if a key is invalid, false is returned.

The compressed syntax on line [#62](#L0R62) can be off-putting to some.  If it's a problem, I'll break it out onto two lines and rebase.

I did run the build through Travis, but I'm an error against [ruby-head](https://travis-ci.org/dacamp/sshkey/jobs/11004091).  Seems like it's a [known issue](https://github.com/travis-ci/travis-ci/issues/1195) though.  Might not be a bad idea to configure ruby-head as an allowable failure.

If you think this addition is useful, please review it and let me know if you have any questions.  Thanks!