handles JavaScript reserved words

[dimitropoulos]

closes #18

It seems like our options are:

1. throw when trying to serialize identifiers that will result in syntax errors
2. do something to the identifiers so that they're not syntax errors

• this will, of course, introduce inconsistencies for special cases where these reserved words have been used
• I am pretty open minded on what the specific suffix should be.

[dimitropoulos]

closing in favor of #28

As it turned out, this functionality already existed in the prodobuf-es codebase, so we just needed to expose it (and thus not reimplement here)