make documentation available at Readthedocs

the first time I had to install DAS it was really frustrating that it was hard to locate the detailed/clear installation instructions.

* added link to online documentation at readthedocs, so users may see it immediately online. it can be rebuilt automatically, on both new commits or releases...
* added requirements.txt file for virtualenv which shall bring at least almost all dependencies

P.S. status: readthedocs build succeeds. still some doc/doctests dont pass e.g. because of issues with pymongo, but I don't think this is important. from what I've seen the generated docs already look OK.