added interpolation to lineChart

Signed-off-by: Jacob Rideout <jacob@jacobrideout.net>
Updated interpolation with tension

Signed-off-by: Jacob Rideout <jacob@jacobrideout.net>
Merge branch 'add_interpolate'

Conflicts:
	dc.js
backport add_interpolate to src.
Closes #233
Merge pull request #266 from NickQiZhu/add_interpolate

Expose interpolation and tension for lineChart