Let cursor add additional information to response.

For elasticsearch facets feature there is additional object returned
with result. If possible for cursor to get the response before it's
send it can add this information.
Use cursor.extra for extra response data.

Instead of cursor.info use cursor.extra attribute to provide
additional data to response.
Fix python2.6 tests.

AssertIn is not supported, use assertTrue instead.
Merge branch 'cursor-info' of https://github.com/petrjasek/eve into petrjasek-cursor-info
[new][dev] 'extra' cursor property.
Merge branch 'petrjasek-cursor-info' into develop