PyConLightningTalkProposal Submission

Adds Support for Lightning Talk Proposals

Discussion Points:
* Inherits from PyConProposal
* assigns default vals to required fields (**category** and **audience_level**) that the end user doesn't need to select from
  * Could not see any negative ramifications to this, but it should be noted.
git 
* Adds fixture data for **ProposalSection** and **ProposalKind**