support HTTP OPTIONS requests

[davidsarah]

secorp was testing the JavaScript webdrive code, and found that it was failing due to lack of support for OPTIONS requests. Here's some code to add to RenderMixin in src/allmydata/web/common.py​ to support OPTIONS:

    def render_OPTIONS(self, ctx)
        from allmydata import __version__
        req = IRequest(ctx)
        req.setHeader("server", "Tahoe-LAFS gateway v" + __version__)
        methods = ', '.join([m[7:] for m in dir(self) if m.startswith('render_')])
        req.setHeader("allow", methods)
        req.setHeader("public", methods)
        req.setHeader("compliance", "rfc=2068, rfc=2616")
        req.setHeader("content-length", 0)
        return ""