Added forking callbacks to edm::one::OutputModuleBase

Since PoolOutputModule still needs the forking callbacks, they were added to edm::one::OutputModuleBase
Have PoolOutputModule inherit from edm::one::OutputModule
Merge pull request #1114 from Dr15Jones/PoolOutputModuleToOne

Have PoolOutputModule inherit from one::OutputModule
Merge remote-tracking branch 'origin/CMSSW_7_0_X' into CMSSW_7_0_X