Add fabfile generation to pelican-quickstart

This commit adds optional fabfile.py generation during the
pelican-quickstart process. Reasons include:

* "make" is cumbersome to install on Windows
* Fabric runs in any Python environment
* fabfile is just Python and thus more flexible and extensible

This is an initial implementation and does not currently provide as many
upload options as its Makefile counterpart.

Refs #584.