Prevent errors in browsers that don't support canvases.

Added feature detection to prevent the plugin from executing if the
browser doesn't support canvases.
Added the `error` hook, triggered if the browser doesn't support
canvases.
Added the `error` hook to the doc.
Merge pull request #82 from pioul/master

Prevent errors in browsers that don't support canvases
"cursor" option description changed

Reading the plugin code I noticed that cursor width could be changed passing a numeric value, so I changed the readme in order to explain this useful feature.
Merge pull request #101 from NinjaTux/master

"cursor" option explained better in readme file
Support for HiDPI screens #99 #93 #61
excanvas support for IE7/8 compatibility

added excanvas support code from @buradleix https://github.com/buradleix/jQuery-Knob/
Merge pull request #110 from fadomire/master

excanvas support for IE7/8 compatibility
font and fontWeight support #100 @kylestetz @asnyder
font and fontWeight support #100 @kylestetz @asnyder
First implem of responsive support #98
fits outer div in height when % #98
input readonly support
fix #118
precond thickness opt #121
Allow dynamic resizing of read-only knobs.
Merge pull request #133 from jwalton/master

fix Allow dynamic resizing of read-only knobs @jwalton
fix issues when the input step is decimal

for example, range is from 0 to 1 and step 0.1
fix issues when the range is negative

for example, -10 to 0 with step 1
add decimal and negative range examples to documentation