reorganize the plugin: use a central image cache and only contact gravatar asynchronously

this should solve all the issues where gravatar plugin blocked or extremely slowed down the loading of jenkins' people-page
clarification in test method names
increase test coverage
increase the unit test coverage
test the gravataruser class
refactor out business classes to be able to test them better
testing the loading cache implementation
better dependency configuration
Merge pull request #3 from simschla/better-handling-of-many-many-users

Better handling of many many users: refactor the caching mechanism