[MONDRIAN-1836] move adomd tests to XmlaAdomdTest
CoalesceEmpty: treat empty strings as null
fix IndexOutOfBounds fetching member by key when all levels are marked not unique
lookup child by key, +SsasKeyLookup: do this by default (no level specified)
use description property on member rows if set
Merge pull request #10 from tgf/evergreen

Key lookup, CoalesceEmpty, Description, tests