Edits to Elections spec following implementation

[gordonje]

We're submitting a few edits to the draft elections OCDEP arising from concerns that came up while implementing this specification in python-opencivicdata.

Most significant changes to note:

1. Election no longer subclasses Event, but with the understanding that a future OCDEP might propose a base Event class with properties shared by Election, Event and VoteEvent.
2. Party is removed as a proposed data type, having no significant properties to distinguish it from Organization.

I've also just tidied up a few things (e.g., moved links to resources to the bottom of the .rst doc, corrected a few typos, etc.)

@jpmckinney @jamesturk @fgregg let us know if there are any questions/concerns.

[jsfenfen]

+1 for making election not a subclass of event

[jpmckinney]

Indeed, looks good :)