Use named module amd syntax

Simply calling `define(paper)` in a r.js build throws a deps error. Switching to the named module syntax fixes this issue.

Libraries such as jQuery and Hammer.js use this syntax for AMD support.
fix typo "Concatentates".
Fix to #263: _children undefined inside call to myShape.isEmpty().
Merge remote-tracking branch 'huochunpeng/fix_typo_in_matrix'
Merge remote-tracking branch 'iamdustan/patch-2'
Merge remote-tracking branch 'duckmaestro/master'