Exports for AMD and CommonJS environments

Following up on @PaulUithol 's comment (https://github.com/PaulUithol/Backbone-relational/pull/215#issuecomment-36019250) about what should be returned by the module, this PR

* returns only what Relational adds to Backbone in AMD and CommonJS environments (see https://github.com/xcambar/Backbone-relational/compare/module_exports?expand=1#diff-c99214ca1078164501c576035b7bd1a0R2062)
* still augments Backbone in all the environments (for backward compatibility) (see https://github.com/xcambar/Backbone-relational/compare/module_exports?expand=1#diff-c99214ca1078164501c576035b7bd1a0R2061)

This change allows AMD/CJS developers to use Relational in a way they're more comfortable with, _ie_ __without__ necessarily putting everything under Backbone.* umbrella.