Create atomic-maven-plugin

  * Move the processing of the atomic annotation to a specific mojo.
  * Closes #78
Move Atomic annotation to core module

  * this eliminates all the indirections in the use of AtomicContexts
  * now, the FF strictly enforces the AtomicContext to execute
    TxManager.withTransaction.  As before, each backend specifies how the
    method withTransaction should behave.
  * Closes #70