Integrate an appropriate UMD pattern

[mikeric]

Rivets.js needs to follow some sort of AMD + CommonJS module definition pattern.

[mikeric]

Added support for CommonJS and browser globals.

[peter-schwier]

Please add two more lines so that AMD modules are also supported.

Change:

if module?
module.exports = rivets
else
@rivets = rivets

To:

if module?
module.exports = rivets
else if define?.amd
define([], rivets)
else
@rivets = rivets