Add pivotGroup to performs a cross tabulation against other already defined groups.
Implementation is simplest thing that can work: it doesn't respect filter changes, doesn't handle data added after first executed, and is not performance optimized
Add test that pivot group respects filters
Add test for custom reducer
Update pivot groups when filters change
Add a larger data set for integration testing
Refactor pivot group construction
Use FNV-1a hash to construct pivot group
Significant performance improvement over brute force search (no surprise).   71ms to group 100k records into a 3 dimensional pivot group with 2800 distinct keys (was taking ~1900ms before with brute force search).
Fix bugs with hash and cardinality 1 dimensions
Merge branch 'pivotGroup'
Recompiled crossfilter to include pivotGroup functionality.

Conflicts:
	crossfilter.min.js