Added AOP Advice Wrappers

Added a collection of implicit conversions that convert advice functions
into *Advice classes usable by Spring AOP.  Based on hekonsek initial
pull requests, with the following changes:

- Renamed MethodAdvice to AdviceConversions
- Removed explicit methods in favor of implicit versions
- Added Scaladoc
Merge pull request #5 from hekonsek/aop_advice_wrappers

# By hekonsek
* aop_advice_wrappers:
  Added AOP Advice Wrappers