Replace rake functions with python paver functions

New version to replace #2065 which replaced #1687

Of interest to @sarina, @singingwolfboy, @jarv, @e0d, @feanil, @wedaly, @jzoldak, @rocha, @mjg2203, @nedbat, @cpennington

I have deprecated the rake functions that paver replaces so far (more to come). The rake function will print a warning, wait 5 secs and then run the paver command. I have deleted the files and replaced them with file_deprecated.rake instead.

Have run through all of the functions that were output by rake -T

So far, now handles all of prereqs, assets, django and docs

Will be working on the remaining rake functions - so version 2 will have all rake functions replaced - grateful if people don't write any new ones?

I have cleaned up the process handling and also prerequisite caching from the initial version.