extents function added to the fc namespace.

This function is similar to d3.extents but takes 2 arguments the data and the field names.
- Data can be an array or data arrays or a single data array
- Fields can be an array of fields or a single field name
The function returns an array of two values denoting min and max.
Removed domainFromValues functions from the scales. these are now superceded by the fc.extents function.
Merge pull request #189 from tunerscafe/master

extents function added to the fc namespace.