python packagisation: Remove misleading "For backward compat" comments

`stbt.get_config` is still the official public API. Splitting stbt.py
into a python package with sub-packages is --so far-- purely an internal
organisation / tidiness exercise.
python packagisation: List stbt public API in __all__
python packagisation: Take APIs to document from stbt.__all__

Instead of maintaining the list in 2 places. The documentation is now
sorted alphabetically instead of thematically, but I think that's
appropriate for reference documentation -- the man page isn't a tutorial.

I had to add a docstring to `ConfigurationError` so that it would
display correctly in the generated file.
Merge pull request #218 from drothlis/python-package-2

Some minor enhancements missing from 8ae1c0c4.
Refactoring: Factor out loading templates

In preparation for adding `stbt.match`.

Internally we now pass around an `_AnnotatedTemplate` object rather than
an `numpy.ndarray` so we can attach the template name and any other
information we wish to. This tidies up an number of bits of code and
should make them more robust to changes in the future.
stbt.py: Refactor drawing MatchResults on frames

We can re-use this when adding the `stbt.match()` API.
WIP: Add `stbt.match`

Added new API `stbt.match` which was previously conspicuous in it's
absense.  It simply searches for a template match in a video frame,
returning a `MatchResult`, and notably not throwing an exception if the
template cannot be found.

This, in combination with the ability to treat `MatchResult` as a boolean
makes it much easier to compose matching operations in user code.  e.g.:

    assert stbt.match('template.png') or stbt.match('template2.png')

TODO:
* Add tests involving passing a frame with the frame= parameter
WIP: API: Add region parameter to match

TODO:
* Documentation
* Tests
fixup! WIP: Add `stbt.match`