Smarter range parsing

It used to return (0, None) when one of the 2 values was invalid (not an int).
Now it defaults the start byte to 0 if invalid, and the end byte to None,
independently.
Added tests for shiva.fileserver.get_range_bytes

And the fix for a couple of issues that the tests found.
Default Sphinx structure
Moved most of the README file to docs/index.rst
Linked the RTD docs
Merge pull request #116 from tooxie/crange

Smarter parsing of the `Content-Range` header.
Broke down documentation into files
Fixed resources documentation
Added documentation badge
Changed slogan
Contributing and Whish List
Merge pull request #117 from tooxie/docs

Documentation
Tracks without album or artist

Now the fulltree for an artist includes a `no_album_tracks` attribute, which
contains the list of tracks for that artist that are not related to any album.
And the `/tracks` resource can retrieve the list of tracks that have no album
(`/tracks?album=0`), no artist (`/tracks?artist=0`), or neither
(`/tracks?album=0&artist=0`).
Orphan tracks documentation
Merge pull request #119 from tooxie/orphantracks

Orphan tracks
Added User model
Added UserResource resource
Explicit assignations and simpler signature
Changed destination path for converted files

The converter won't pollute the directory where files are originally stored any
more. Now it creates a sub directory named after the file extension and stores
the new files there.
Handling the case of filenames starting with a dot