Updating the PyCon API.

Note: This is a BACKWARDS INCOMPATIBLE commit.

This commit updates the PyCon API drastically:

  * It alters the URLs
      * The API key is now sent as a header, rather than part of the
URL itself.
      * URLs now respond to GET and POST, with POST being intended for
writing, and GET for reading.
  * Requests to the API must be **signed** using a secret key that is
issued as part of an API key.
  * The proposal detail endpoint actually shows details from proposals,
rather than only what was set using the API. Arbitrary data can still
be set and comes down under "extra".
Better filtering in the proposal list view.
API requests now require a timestamp.

API requests now require a timestamp in the header, and in the request
signature, thereby expiring requests after a short period of time.
CSRF Exemption for API methods.
Improve benefit column titles in sponsor admin
Change some button colors

Changed button classes to btn-primary (blue) or btn-success (green)
in a few places:

* Login page: blue "Log in" button
* Save password change: green
* Save changes button when editing box: green
* Submit button on financial aid and proposal review pages: green
Changes based on @dpoirier's comments.
Moving around imports in one other file.
Remove exception catch. >= 500 responses will come down as HTML.
Merge pull request #110 from caktus/feature/110-button-styling

[Settings] Normalize Settings vs Change Password Save button
Merge pull request #204 from caktus/feature/204-benefit-names

Sponsorship benefit admin tweaks
Merge pull request #195 from lukesneeringer/develop

API Revamp
Merge branch 'develop' into staging