Asynchronous transpiling fix

[apowers313]

Related to issue #12

Changes:

• The previous fix would sendTask twice with asynchronous code. This adds sendTask to an else statement in _runNow so that runs in either the transpiling OR if the transpiler isn't run.
• Uses a Promise if it exists; otherwise does the try / catch inside a polyfill. Should make the code easier to upgrade if / when bumping up to a new version of node.js; and makes it easier to read in the meantime.

[n-riesco]

This return should ensure sendTask is run only once. If you're really convinced this is happening, I'll try to reproduce the issue, but I need to be able to reproduce it, so that I can write a test.