Use custom class loader to load annotations from pool

This small change enables to load annotation properly when loading a class that contains an annotation. 

https://github.com/jboss-javassist/javassist/pull/18

It will definitely make it much simpler to use the plugin in android as every derived plugin will be able to load annotations from the dependencies of the project where the plugin is applied. Without this change, annotations would have had to be loaded both in the project itself (to compile) and added  to the buildScript scope (to be known by the plugin).

@darylteo , do you think you could release version 0.4.1 soon ? (I increased version number in the PR already)