Add a GNU make build system

This adds a basic Makefile to the project. Currently the Makefile has basically two targets:

- `install` – This target creates a virtualenv and installs the project's dev egg in the virtual env. It also creates the symlink for mapnik. (Although it's fragile for now because Python 2.7 is assumed.)
- `check` – This target runs flake8 and nosetests.

I'm creating this PR for discussion, and to know if it makes sense for the project.

Note: the .travis.yml file could be updated to use the Makefile.