Bump to 3.1.6, add seafile-server ebuild

Hi!

I bumped the ebuilds to 3.1.6, changed several things and also added a new seafile-server ebuild.

Since it is kind of horrible to build the seahub webinterface with Python system packages I decided to use the official seafile-server tarball and simply replace the included binaries with the ones built by the ebuilds.
I don't really like it this way, but I had hours of problems combining the dependencies mentioned in the official docs.
For example: They state they need djblets-0.6.14, but that version is actually incompatible with Django-1.5, which they also need. There are also lots of problems with pillow/PIL. I played around with creating a virtualenv inside the ebuild, but that felt even more hacky and strange.

The distributed seafile-server tarball seems to work fine with virtual/python-imaging, python-2.7 and the binaries installed by the libsearpc, ccnet and seafile ebuilds. I tested the webinterface and the Windows client with an amd64 Gentoo installed in a VMware VM.

I'd like to hear some feedback on the ebuilds I created, mainly because they work different than the old ebuilds (net-misc/seahub isn't needed anymore, for example).

Things that I didn't do:
- didn't bump or test the seafile GUI and CLI clients
- seafile-server won't work on x86 and will probably needs some changes (SRC_URI and libs cleanup, for example)