Added a shared cached dummy canvas context that may be used by Kinetic.Text methods.

This greatly improves performances when creating lots of Kinetic.Text nodes, especially on mobile devices.
refactored the whole _setTextData method of Kinetic.Text, added a 'wrapping' option.

Performances should improve.
Also, the text should not overflow anymore from the defined width like it sometimes did.
merged and tweaked pull request 314