Export the Twitter symbol explicitly for use in Meteor 0.6.5 and above

[jordn]

Meteor 0.6.5 requires pacakge exports to be declared. Added in a way that should be backwards compatable with pre-0.6.5 too.

"Since Geoff's big email, we simplified the way that exports from packages work. There's no more magic comment scanner: you just declare exports in the on_use function with

api.export('MySymbol', 'server'); " — https://groups.google.com/forum/#!topic/meteor-talk/YOStFiuzbO8