Published documentation should be version specific

[vanpuk]

Copied from inasafe/inasafe#268

I'v just been reading the sentry docs and notice they use a schema like this:

http://sentry.readthedocs.org/en/v1.13.5/install/index.html
and

http://sentry.readthedocs.org/en/latest/index.html
For their documentation and was thinking it may be good to do the same - so that as we release successive versions of the software to older version's documentation will always be available, and also that we can separate master / in development docs from current stable release's docs.

I don't think this is a huge priority for right now, but I'm adding it here as a 'nice to have' idea for the future.

Regards

Tim