Implement Iterator protocol on Repository

Uses a stupid implementation that builds a list in memory and returns
it's iterator.
Ensure that odb is always freed
Handle errors while building the list gracefully
Don't leave dangling references to oid strings
Always decref the new string. Don't set err.
Shamelessly add richo to AUTHORS
Use correct prototypes
return NULL over -1
Merge remote-tracking branch 'richo/bugs/compiler_warnings'
Make Repostirory_as_iter shorter

Error_set already returns NULL
Readme: add table of contents
sphinx-quickstart
Use autodocs
Copy docs from the readme file
Simplify README file
Update authors list

And remove AUTHORS text file.
docs: split usage guide into several source files