A number of tweaks, improvements, and fixes

- Fixed a bug that raised an error when using the drawText() method
- All jCanvas methods now respect the value of the 'visible' property
- Graph functions, drawn using drawGraph(), can now access the given
parameters object
- There is now only one supported syntax for defining function layers
(using the 'fn' property).
Fixed a bug affecting the 'type' property
Merge branch 'master' of https://github.com/caleb531/jcanvas