Allow more general web based job scheduling

[emin63]

Need a dynamic way for users or plug-in writers to create ways to schedule jobs.

Basically we need plugins or users to somehow tell ox_herd at run time that they have a new kind of job which can be scheduled and point to the WTForm instance for that.

Then ox_herd should list all the possible jobs it knows about and let users choose from those jobs, provide the appropriate form, and then create the job and schedule it.

[emin63]

I think we need a plugin architecture for this.

Plugins should work as follows:

1. Follow conventions in ox_herd/core/plugins.py (note: I need to write plugins.py).
2. Indicate that they should be enabled. This can take one of the following forms.
   1. Provide a --plugin option in ox_herd/scripts/serve_ox_herd.py script.
   2. Add name of plugin to OX_PLUGINS variable in ox_herd.settings.py.
      1. This can be done at run time if another application is using ox_herd as a flask blueprint.

Basically a plugin should be the name of a python package which can be imported using imp. The imported module should provide a get_plugins function that returns a list of sub-classes of OxPlugin. The main ox_herd app will them initialize instances of those classes and periodically ask them for things like forms they provide for commands to schedule jobs, and so on.

Note that plugins can also override or add to available jinja templates to help with flask.