Support directly passing key and certificate rather than loading from file

[pnc]

We make multiple connections for a couple different iOS applications, and so we store our keys in the database rather than on the filesystem. This change lets you specify a DER-encoded key and certificate as part of #apns_connection's options, mimicking ssl:connect's options.

Another way to do this would be to directly support setting ssl:connect options in #apns_connection. If you prefer this, let me know!

This fixes #26.

I've left this as working commits for easier review, but I can squash prior to a merge.

If you plan to merge this, I'd like to add a snippet to the README about how to use Erlang's public_key module to decode keys into the proper format for use with these new options.

[elbrujohalcon]

@pnc looks good, can you update README.md accordingly, please?

[pnc]

@elbrujohalcon Let me know if that looks good!

I'm exposing the fact that ssl:connect wants the binary key as {'RSAPrivateKey', binary()}. If we want to assume Apple will only accept RSA keys, we could hide this detail inside ssl_opts.

Have you ever tried submitting a DSA signing request to the Developer Portal?

[elbrujohalcon]

@pnc no, I have not. But, I don't mind exposing that detail. I'll just merge the pull request