CI setup

[asteinh]

Some reasonable CI tasks:

• unit testing
• testing against Octave versions (testing against 4.4.1 and 5.1.0 on Debian x86-64)
• testing against TensorFlow versions (no benefit, test for latest only)
• binaries + releases
• documentation (on pages)
• packages: Docker images with Octave/tensorflow.m pre-installed

[asteinh]

Docker images with Octave/tensorflow.m pre-installed are being built on release (see Circle-CI config). However, I'm not sure if this is gonna be used any further at this point... focusing on binaries for Octave 4 and 5 now.