Allow arbitrarily nested HTTP directories to be expressed in URI to retrieve "index.html"

The URL can omit explicit "index.html" after a subdirectoy in the HTTP path, e.g.:

    http://domain.tld/sub/another/
    http://domain.tld/sub/another

Both URLs -- with our without trailing slash -- request the following
static resource from the phantom-api web server:

    DOC_ROOT/sub/index.html

Also, allow developer to define the DOC_ROOT value in the
configuration with or without a trailing slash; phantom-api reconciles
these two styles so the developer isn't punished for such a trivial
keystroke in this context.