VoteEvent: Add requirement attribute

[jamesturk]

Different kinds of votes in different legislatures require different percentages of support to pass. Seems like important information to store about a vote - more important than shoving it into extra attributes. The @opencongress congress scrapers include this attribute in votes as you can see in the following example.

{
  "requires": "1/2", 
  "result": "Failed", 
  "result_text": "Failed"
}

The result_text may also be relevant too since we're storing passing as a boolean value instead of the actual text specific to the vote type. This may make more sense though to push to extra attributes.

via @crdunwel opencivicdata/python-opencivicdata#38

[jpmckinney]

The Motion class in Popolo has a requirement field: http://www.popoloproject.com/specs/motion.html