Static document pages with travis fix

Reapply https://github.com/EUDAT-B2SHARE/b2share/pull/352 with a fix for Travis.
Added a structure for displaying markdown (wiki) documentation as described in ticket: #326
NOTE: a new dependency: markdown is added to `requirements-b2share.txt`

Travis did not build requirements for b2share, wait for the green Travis-CI build before merging.