Fixes bug 497.  Changed baseMixin filterHandler to correctly handle filtering dates.  We use a trick from Crossfilter 'a <= b && a >= b' instead of 'a == b' because the former coerces the values to primitives which then works for dates.
Minor fixes to removeFilter and hasFilter that are congruent with the changes in filterHandler.  filters may be dates, thus indexOf will not work.
Merge branch 'master' into fix_bug_497
Fixes row test case that was comparing key values of the SVG to the filters.  The key is numeric and not a string.
Merge pull request #555
fix test name
artifacts
add test for date filtering failure