1063311 - admin should be able to delete task
add copr modification to web api

I've reviewed the issues raised in the code review:

1)
copr = coprs_logic.CoprsLogic.get(flask.g.user, username, coprname).first()

Really shouldn't need try/except wrapping, looking at the source it just returns
None when the username doesn't exist, which is handled correctly, and empirical
testing confirms that.

2)
if form.description.raw_data and len(form.description.raw_data):
   copr.description = form.description.data
etc.

Really does handle the optional field as it should, meaning you can delete e.g.
description by passing an empty string in POST, and on the other hand when you don't
pass the field at all, it will not be changed.

3)
I've changed the chroot modification URL to /coprs/<username>/<coprname>/detail/<chrootname>/
as requested.

4)
Added the API documentation.
add initial version of dnf plugin
fix compilation problems
add get_summary and get_usage

addressing:
W: 22, 0: Method 'get_summary' is abstract in class 'Command' but is not overridden (abstract-method)
W: 22, 0: Method 'get_usage' is abstract in class 'Command' but is not overridden (abstract-method)