Add geocoding to directions.inputControl

[tmcw]

closes #73

[jfirebaugh]

Here's what I suggest for the remainder of this method:

if (err) return cb(err);

try {
   resp = JSON.parse(resp.responseText);
} catch (e) {
    return cb(e);
}

return cb(null, resp);

So:

• Pass the corslite or parsing error directly to the callback, reserving this.fire('error') for actual direction request errors.
• Let the callback receive the full response. It can take care of extracting the first result, or showing a list of results if desired.

[lbud]

@jfirebaugh I think this is ready — mind taking a quick pass before merge?

[jfirebaugh]

It should be safe to remove the || fallback entirely now.

[jfirebaugh]

This looks good from a code standpoint but I'm concerned about issues like https://github.com/mapbox/geocoder-feedback/issues/60 -- if I'm zoomed out looking at California and search for "San Francisco" to "Chico", I'm not expecting to start from "San Francisco St, Yountville, 94599, California, United States".

@ingalls, anything you can suggest to get more predictable results? Setting the precision of the proximity parameter based on the map zoom doesn't seem to have the effect I would hope for.