Generate valid clip path id

Prior to this patch, if you created a dc chart using a complex selector
like dc.barChart('#area .bar'), the resulting clip path id wa invalid:
'area .bar-clip'. This patch replaces spaces, periods, and hash marks
in the id with hyphens to generate a valid clip path id.
Merge pull request #685

Conflicts:
	src/coordinate-grid-mixin.js
add test for PR #685

@jcamins, adding a simple test for your PR on clip path id
to make sure it stays fixed and to show the intended behavior
please check and suggest improvements, if any.  Thanks!