bugfix (#56): Clear background completely on redraw

When redrawing the canvas, and the background was set, the
background was redrawn on top of the old drawing, without clearing.
If the background was partly transparent, that would cause the old
frame to still be visible.
(feature): Add pieSection option to arc objects

This will allow you to create pie chart sections without resorting
to the stroke solution available before. This uses fill instead,
and might render better in some browsers.
(bugfix): Fix incorrect color parsing for strokes

The parsing of stroke colors failed when the color was a gradient
with color stops containing positions.
(bugfix): Fix stroke gradient parsing for colors with parens

When specifying a gradient where the color stops are colors in
formats where parentheses are used, like rgb(), the parsing needs
to handle the multiple levels of parentheses, for both the
gradient itself, and the color stops that it's made up of.
(feature): Add support for pixels in text lineHeight

Text line-height can now be set using pixels, instead of only
relative to the font size. It works both in short-hand syntax
12px/18px sans-serif
and specific property setting
text.lineHeight = "18px"
as well as getting the value
text.lineHeight; // "18px"
Relative values will work just as before.
Update version number to 2.3.0.