Update for OS X Mavericks

[rgov]

I updated the event handler script to support Messages on OS X Mavericks. The changes include:

• Change from binary-format .scpt files to plain-text .applescript files
• Rename the AppleScripts to follow more typical naming conventions
• Rewrite installation instructions according to new Messages UI
• Add unimplemented functions to the event handler script to avoid errors

[lazerwalker]

Awesome, thanks!

Do you know if these changes will still allow it to work on Mountain Lion, or are the plaintext .applescript files Mavericks-only?

(I think Mavericks-only is fine, but if that's the case I'll add a note about that in the README and perhaps tag a previous SHA as having legacy support)

[rgov]

I don't believe that .applescript files are new to Mavericks, but the Messages event handler system might be different (such as now using a single script file, instead of many).

[lazerwalker]

Alright. I don't have a good way to test for earlier versions of compatibility, so I might just make a note anyway, since that's pretty low-impact.

Thanks again for doing this! Clearly I haven't been using this lib lately, so it's greatly welcomed to see someone else help whip it into shape :).