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

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