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
Bumping scalatest version; removing publishTo; excluding scalatest from assembly jar config since it was breaking builds
Setting correct host header with hostname and port
Fixing overlay scaling by basing it on layer size, not final image size
Conditionally scaling image to fix dimensions: we do this because the Bicubic scaling blurs the image slightly even if the size does not change
Using correct resources path
Add .travis.yml for travis builds
Merge pull request #9 from endenizen/add_travis

Add .travis.yml for travis builds
Add build status to readme
Merge pull request #10 from endenizen/update_readme

Add build status to readme
Reducing minimum image size to 1x1
Merge branch 'master' of github.com:rdio/thor