Moved concatenation based dframe method from TableStack to Stack

For real support for exporting dframes from Stacks all the View
dframes receive the Stack dimensions and are concatenated.
Fix plotting of Grid elements which don't support joint_axes
Added interface module with Pandas dframe compatibility wrapper
Added dframe export method to Curve
Fixed bugs in DFrameView export machinery
Added DFrameOverlay and associated plotting code
Added autocorrelation_plot option to DFramePlot
Speed optimization for split_dimensions operation
NdMapping slicing operations now performed iteratively for speed

Previously slicing operations would generate all the filter
conditions and apply them all at once. This commit switches to
an iterative approach, where each condition filters the list
of items incrementally.
Added dim_values method to NdMapping
Cleaned up imports in interface module
Switched .iteritems to .items in NdMapping for Py3 compatibility
Return list rather than set from dim_values on NdMapping
Enable passing of mode (set/merge) via kwargs in Collector.analyze
Improved index selection in DFrameView _export_dataviews
Added bins option to DFrameView hist plot_opts
Formatting and docstring fixes in interface module
Improved dimensions handling in DFrameView constructor
Enabled <= ndim slicing in DFrameView getitem
Fixes to DFrameView dimensions handling