Added ability to user gradients on bullet charts.

While using bullet charts, you now have the extra option "gradient"
which can be set to true. If it's set to true, a gradient spectrum is
created for the rangeColors that are passed. If only 1 color is passed,
all ranges are that color. If more than 2 colors are passed, the
gradient shifts between each color. This has been implemented with
Rainbowvis-js (https://github.com/anomal/RainbowVis-JS). It has been
included and linked in the makefile.
Added support for devices with a devicePixelRatio > 1, like all devices with so called "retina" screens.
Merge branch 'master' of https://github.com/khenderick/jquery.sparkline
Merge remote-tracking branch 'remotes/ericburns/master' into takacsv-work

Conflicts:
	Makefile
added rainbowVis as a submodule
keeping the external rainbow-vis library out of the concat build: it should be loaded separately when needed anyhow
Merge remote-tracking branch 'remotes/tnt-dev/master' into takacsv-work - retina screens support