New API command: getInboxMessageById

I have added a new API command: getInboxMessageById. This method allows a user to input a known message ID and retrieve the related message details. The details returned (in no particular order) are:

1. encodingType
2. msgid
3. toAddress
4. fromAddress
5. receivedTime
6. subject
7. message

The getInboxMessageById method takes an input parameter of <msgid> which is encoded in hex, just like the getAllInboxMessages function.

This pull request is my first go at adding a new API method to BitMessage. I plan to try and add support for getNetworkStatus next.