Use node-mapnik for blending

[springmeyer]

• Moves to using node-mapnik@1.4.9
• Because node-mapnik offers packages with windows binaries with opens the door to abaculus running on windows
• Added an appveyor config so that, like travis, commits will be tested and viewable at https://ci.appveyor.com/project/Mapbox/abaculus.

Currently the tests are failing on windows (no surprise) and the actual errors are not visible (fun!). They are not visible due to an appveyor/windows/node.js limitation that can only be worked around by using Node v0.11.x.

So, next steps after merging:

• [x] @springmeyer gets node-mapnik packaged for windows against node v0.11.x
• [x] Then we can consider trying to get abaculus tests passing on windows since we'll actually be able to see the errors in the appveyor logs

[springmeyer]

tests are now passing on windows, merging.