Add getGeneLocation webservice method

Given a gene symbol and optional genome build, this returns the location
of the gene.

Primary motivation for this is LOVD, where it will be used in combination
with sliceChromsome as an alternative for sliceChromosomeByGene which only
works on the fixed Ensembl genome build.

Fix error in git tag instructions

Release 2.0.5

Update documentation homepage layout

Update documentation on contributing and releases

Compilation of developer documentation on Read The Docs

We use a separate `doc/requirements.txt` file for building the
documentation. Since on Read The Docs where we cannot install
packages depending on C libraries, we mock these packages in
`doc/conf.py` and omit them from the requirements.

Add Travis CI configuration

See merge request !26