This will allow to use ES2015 constructs in spec code.
The change required introducing a small server, which serves both PEG.js
and spec code passed through Babel and bundled together. This allowed to
convert the specs to regular modules and get rid of the hackery that was
previously needed to make them run both in Node.js and in the browser.
Note the specs no longer exercise the browser version. This will allow
to spec PEG.js internals in the future.
See #442.
