Separated parameters for message body & message properties

[EvanK]

This introduces some minor backward compatibility breaks, but in my opinion the bc break is worth the perceived improvement in usability. The bc break, however, would probably make bumping the version

Notes:

• Explicitly casting body to string in IronMQ_Message#setBody. Alternatively could json_encode message bodies before posting and json_decode after getting, which would allow & preserve most non-string and even non-scalar typed data.
• Changes to IronMQ#postMessages allows the posting of multiple message bodies with identical message properties applied to each. This seemed to me a value-added feature, but one could just as easily accept an array of properties arrays to instead preserve the original behavior.

[thousandsofthem]

Sorry for extremely long delay. New syntax totally reasonable and handy, thanks!

[EvanK]

Always happy to help