DevelGuide updated + some changes in the source.

I found some time to update PyHumod Developer's Guide. The wikidoc has been extended by the event handling section. I also worked on the source code in the mercurial repository and today's changes include:

• deprecation of *_pbent, all SMS-related and entering Text/PDB mode commands. I will update the documentation reflecting it as soon as pyhumod-0x03 is available for download. I must admit I didn't put a lot of thought initially into the naming of the methods and they were a little random. From version 0.03 on, all SMS related Modem() methods will start with sms_* prefix, the phone book related ones with pbent_* prefix and so on.
• Code cleanup to avoid repetition. See _common_enable for details.
• Enabling CLIP (Calling line identification presentation). This will be useful in conjunction with event handler, as it tells you who (what number) is actually calling you.