Multiple index file names

Recognising index.html or alternatively one other index file name is certainly useful, but there are many cases in which other conventional index file names are used. This PR makes it possible to specify an array of such names that are tried in turns.

I'm using send to serve a variety of sites from the same code base, notably the Nightly build of the HTML 5.0 specification[0] which in accordance with age-old tradition uses "Overview.html" for its index page, and this change is pretty useful for that scenario.

[0] http://www.w3.org/html/wg/drafts/html/master/Overview.html