Pybars3

- Changed `pybars.__version__` tuple to `pybars.__version_info__`,
`pybars.__version__` is now a string
- Added `Compiler().precompile(source)` that will return Python source code
to allow for caching of compiled templates and easier debugging
- Template code now checks to ensure it is being run with the same version of
pybars that is was generated with

- The library now always throws `pybars.PybarsError` on errors
- Added support for nested subexpressions
- Block helpers by themselves on lines no longer introduce any whitespace
- Added `tests.py` test runner and `--debug` flag
- Moved tests out of `pybars` namespace

- Fixed a bug with nested scopes and parent paths (`../`)

- Expose `pybars.Scope()` so helpers can properly scope data so `../` will work

- Added subexpression support

- Changed `False` to print `false` and `True` to print `true`
- Added support for `null` and `undefined` literals