Guarantee unique temporary variable names.

Closes #11.
Closes #22.
Require recast v0.6.5 (for ast-types v0.4.7).
Bump NPM version to 0.4.11 in preparation for publishing.
Reenable node v0.6 tests (no longer allow failures).
Make bin/regenerator read from STDIN when no file name given.

Closes #125.
Bump NPM version to 0.4.12 in preparation for publishing.
Support generator.return(argument) method for closing generators.

Closes #107.
Add a basic test of generator.return(argument);
Make sure generator.return(argument) works for newborn generators.