Moved backend classes to sipsimple.configuration.backend

Ignore-this: 46eb39745caac5bf208419b0af763717

darcs-hash:20090316142918-1e3d8-fa6118a7f771c0aa38efdf046592e67ef554390f
Added check for backend type in ConfigurationManager.start

Ignore-this: 3bff28bf6e3db3b65022801909c819b1

darcs-hash:20090316142934-1e3d8-0a4118a9079a4f2a0ea1836482fb3345c4bb453c