externalize processStaticMap -> abaculus -> mapnik

[indus]

I had the same build issue like #66.
After having a look what mapnik is used for it seems to me that I won't use the "staticMap" functionality and therefor wouldn't even need abaculus and mapnik.
Because of the issues that might arise from including a dependency with a build step, the size of this dependency and the uncertain use of the functionality I would wish that it gets provided as an optional extension or plugin.

[mojodna]

Agreed; I just created lambda-tileserver for a related reason. Would you be willing to put together a PR stripping out functionality that uses Mapnik?

It's straightforward enough that another tile server could be easily assembled that includes tessera and adds the static map generation functionality; see https://github.com/stamen/toner-carto/blob/master/server.js for a server that wraps tessera w/ error reporting capabilities.

[indus]

Removing these parts should be easy and I can make a PR for that but building a plugin, witch would require a new repo (?), Is more complicated...

[mojodna]

Published as v0.14.0. I added a note to the README recommending people use ~0.13.1 if they need static map functionality. If sufficient changes are made to tessera, we can revisit creating a server / plugin that wraps it, but for now, that seems like a reasonable solution.