Fixed three bugs.  1. Strip multiple 'send to controller' actions from an OpenFlow action sequence.  2. Add all modification fields to NetCore_Action.Output.domain function.  3. Fix NetCore_Semantics.eval, which was applying the first action twice in sequential composition.
Added concrete syntax for more modifications and updated the manual.  This closes #56.
Merge branch 'master' of https://github.com/frenetic-lang/frenetic