Adding font support

Summary:
Adding font support and new text filter which draws centered text by default
Vertically centering text by ascent
Unsealing trait to allow for extensions
Separating percent filters from pixel filters so we can base percent on layer size, not overall size
Removing constrain/composite stubs

NOTE: This is open-source Thor.

Test Plan:
Manual. Tests pass.

To test yourself, change src/main/resources/reference.conf to point to our media origin:

IMAGESERVER_MEDIA_HOST = img-origin.rdio.com
IMAGESERVER_MEDIA_PORT = 80

Then `sbt run` and browse to:

http://localhost:8080/?l=album%2F9%2F3%2F0%2F0000000000003039%2Fsquare-600.jpg%3Atext(%22TEST%22%2C%20128px%20%22Helvetica%22%2C%20rgb(0%2C%200%2C%200))&w=300&h=300

Reviewers: joseph.norton, jesse, rebecca.stecker

Reviewed By: rebecca.stecker

Differential Revision: https://phabricator.rdio.com/D11736