Implement #295 - Visually separate private from public modules

[mottosso]

#295

This makes many small changes across many files which will make it tricky to dive into each individual change.

In a nutshell, this is meant to prevent accidental use of private members. All tests have been updated to reflect this, external projects such as pyblish-qml and pyblish-lite must now do the same.

A majority of members were already present in api.py, some were not, most notably some in util.py.

The members used in util.py is meant to stem from a public origin, so it's likely these will need to transition into api.py, as "developer extras" or "advanced" functionality, as they will be meant solely for custom publishers and GUIs, and not general consumption.

Still have to think through how to properly expose these, without locking too much of the internals down from changing.

To give this PR a try

$ virtualenv pyblish-base-test --no-site-packages
$ pyblish-base-test\Scripts\activate  # or Linux-equivalent
$ pip install git+git://github.com/mottosso/pyblish-base.git
$ python
>>> # start playing