Capture / save contributions for analysis

[shannonturner]

By saving contribution information, I can then have a page for each company showing all of the politicians (quotes) they've supported with their campaign $.

Will need to create a new model (Contribution) and modify existing model (Contributor) to accomplish this.

[shannonturner]

Will need to be careful to not have duplicates of contributions, and to avoid double-counting if all-time and current-cycle contributions are shown.

Probably best to only save all-time contributions between a company and candidate, and to overwrite values when they change instead of having a new entry.