make Monitor a special kind of Callback, to simplify it's implementation and make it more powerful as well

* some tweaks for travis

First commit for rewriting the unittests when using travis-ci. It
uses unitests from python instead of custom framework and adds
caching to the tensorpack_datadir which path is explicitly set.

* add infogan example to tests

* add flag to create data dir only if not exists

* add several tests including ResNet-18 on fakedata

* fix matplotlib import

* fix python3 bytes-like str

* override image paths in resnet-test

* try to fix ilsvrc paths

* python3 fix for decode str

* fix meta-data path

* next travis fix, arg in python call

* use fakedata in resnet

* fix lint, fix arg

* fix linting in test_resnet.py

* cosmetic changes

* add data_format for resnet-eval

* delete resnet directory

* fix linting :(