* Adds the ability for destination plugins to be sub-classed from ExportDestination. These plugins have the extra option of specifying an export plugin before the destination receives the data. Closes#807.
* fixing tests
This way IDEs can verify method overrides in subclasses, otherwise these
are flagged as erroneous.
Changed base classes to properly raise NotImplementedError; previously
they would cause "TypeError: exceptions must derive from BaseException"
Also fixed exception handling in sources.service.clean().
Mostly typos, grammar errors and inconsistent indentation in code
examples.
Some errors detected using Topy (https://github.com/intgr/topy), all
changes verified by hand.
Jasmine Schladen
Curtis Castrapel
Hossein Shafagh
kevgliss
Marti Raudsepp
kevgliss