Rename XWiki.Configurable to XWiki.ConfigurableClass to conform to naming standards.

After writing XWiki.Configurable I was informed that I should have used the name ConfigurableClass which conforms to the naming standard for XWiki class documents. Sorry guys.
This will affect users who copied out XWiki.RegistrationConfig to save their configuration during an update, The configuration will still work, but it will nolonger be configurable from the administration application. One solution is to export RegistrationConfig, open the xar file in a zip program, edit the file RegistrationConfig.xml inside and change all instances of XWiki.Configurable to XWiki.ConfigurableClass (there should be 2).