Added additional OSM commands

This commit adds commands to import/download from overpass API, export to OSM formats, and export diffs as changesets
It also adds functionality to create mapped datasets from OSM data, export them to shp/pg/sl, and unmap them

It modifies the ExportOp and BranchCreateOp to add some extra functionality that is needed
Merge branch 'osm' of git://github.com/volaya/GeoGit into volaya-osm

Fix tests that access RevFeature attribute values by index to match
the expected value indices after the merge of f5557ce8 where RevFeature
and RevFeatureType maintain the original FeatureType attribute order.