style: consistent message for "0 documents in database"
+func: sortStore contains both lowest and highest value

This enables retrieval of either the low or high value, making results of
ResultSet ordering in ascending/descending order more intuitive. i.e.
when sorting in ascending order the lowest value should be used, when
descending, the highest.
style: `indexStore` PEP8 fixes
style: `resultSet` PEP8 fixes
+func: use low/high value when sorting ascending/descending

When ordering ResultSets by Index, use lowest value for ascending
order, and highest value for descending order.
docs: sphinx-ified `SimpleResultSet.order()` docstring
docs: update CHANGES file
+func: more intuitive ResultSet ordering by Index

More intuitive ResultSet ordering for ascending vs. descending sort.
Indexes with a `sortStore` setting store both the lowest and highest
value. ResultSet ordering using the Index can use the low value when
sorting ascending and the high value when sorting descending.

Merge branch 'feature/sortStore' into develop