Constrain selectors to pivot table context

Fixes issue #34
compiling changes from pull request #35
Update ReadMe.md
Update ReadMe.md
fix for #38
Merge branch 'master' of github.com:nicolaskruchten/pivottable
safer function check
workaround for browsers without Object.keys
table input example
double-click filter now works with numerical attribute values
overridable menu limit
fractionOf aggregator template
fixing label of fractionOf
stacked column charts
general fix for #70
reverting example
custom aggregator example
error handling in pivot() (#24)
error handling in pivotUI() (#24)
typo