add group and aggregate functions
fix parameters on group and aggregate to match pymongo
failing test for reduce
skip the new group and aggregate tests if execs and pymongo aren't installed
Merge pull request #1 from texuf/master

fix parameters on group and aggregate to match pymongo
Refactor reduce_ctx in group, simplify group, remove ungrouped key, value pairs from group results.
Remove initial parameter from reduce_ctx call in group()
remove bad merge
initialize reduced val to proper value
make the group/agregate functions inherit from _CollectionComparisonTest, write more broken tests
Merge pull request #2 from texuf/master

small fixes and more broken tests
Fixes pushing initial values to documents in collection, new tests run properly.
Remove print statements from group function
Removes debug code from tests
Finalize group and aggregate.
touch readme
Fix python3 failure.
Merge branch 'master' of https://github.com/jeffreybrowning/mongomock
Python 3 fix attempt.
Attempt fix python 3 bug.