python packagisation: Move stbt.py to _stbt/__init__.py

This is the first step in converting stbt from a python module into a
python package. I've only moved stbt.py to _stbt/__init__.py; to make
rebasing easier I'll make any necessary changes in separate commits.

The package directory is called "_stbt" instead of "stbt" so that it
doesn't clash with the "stbt" executable program (which is a shell
script). So we need a "stbt.py" module that imports everything from the
"_stbt" package, so that users can continue to "import stbt" instead of
"import _stbt". This is a pattern I copied from pytest:
https://bitbucket.org/hpk42/pytest/commits/4e2a0da
python packagisation: Fix relative paths
python packagisation: stbt.py can't export _stbt.__init__._display or _debug_level

"When you import variables from a module the names are imported into the
current module's namespace, they *are not* shared between both modules."
-- http://stackoverflow.com/a/8393358/606705

This might break some advanced users' scripts, but this was never part
of stbt's public api.