Pypickup

Added/modified features:
- Commands 'add' and 'update' simplified to just one: 'add'. Comand 'update' is no longer available. This fulfills most of issue 25.
- New command 'rebuild-index', which rebuilds either the general or the specific indices for packages, based on the current packages actually downloaded. Useful in case some index has been corrupted or is not coherent with what is currently in the local machine (issue 35).

Added features:

- Command 'list' now shows packages and subpackages in alphanumerical order (issue 34)
- Command 'list' now shows all the available packages in the remote with the option -r, with no filters whatsoever (issue 37)
- Commands 'add' and 'update' now accepts a version specifying, e.g. `$ pypickup add numpy==1.8` (issue 36)
- Commands 'add' and 'update' now accepts a requirements-like file (like the one outcomed by 'pip freeze') (issue 31)

Add tests and fixed dependencies for PyPI.

**Full Changelog**: https://github.com/UB-Quantic/pypickup/compare/0.1.2...0.2.0