Adds test stubs for query tests
Fleshes out basic query tests (failing)
Adds custom exceptions for .find_one()
.find() now accepts query lists of zero length
Adds simple tests for queries
Adds simple tests for comparison operators
Adds simple tests for string comparison operators
Creates simple query and comparison operator tests for mongo.

MongoStorage modified to raise exceptions if 0 for >1 results
are returned from MongoStore.find_one().
Removes .find_all() method from all storage objects.
Completes addition of mongo backend to existing query tests
Stylistic refactor - no substantive changes
Refactors all tests to use dynamic backend loading.
Adds tests for logical operators (AND, OR, NOT)
Merge pull request #29 from lyndsysimon/feature/query_tests

feature/query tests