Rst2dep

Added chain algorithm and signal support

* Chain currently only supported for const -> dep
* Attaches multinuc children to their previous sibling instead of the leftmost sibling

This is a bugfix release

* Fix imports

Initial release as a python module, installable from PyPI via:

`pip install rst2dep`

and runnable as a module once installed, for example:

`python -m rst2dep -p -f rs3 example.rs3`

See README for more options.