Rename Command to Task
Move Method to its own module
Store parameters on Method as JSON field
Rename NodePetriMixin to TaskPetriMixin

It isn't used by dag so the name was too general.
Simplify mixin hierarchy

Originally it was meant to be utilized for both node types, but as we
switch to just tasks it can be simplified.
Allow method to generate its own petri
Reduce some duplication
Allow Methods to customize execution as well
Make ShellCommand subclass of Method
Start requiring 'service' in method declaration
Start requiring 'parameters' in method declaration
Make sure old modules aren't imported
Remove restriction on branches for travis-ci
Merge pull request #21 from davidlmorton/dsl-update

Dsl update
move base method into methods package
embed parallel task mixin into parallel task class
embed shell command mixin into sc method class
Merge pull request #22 from mark-burnett/collapse-mixins

Collapse ShellCommand and Parallel Task mixins
Renamed event to callback
Don't try to instantiate base sqlalchemy class