Added the loading property to google-map-search.

[tekkamanendless]

I try to minimize the amount of Javascript that my elements need to function, and the lack
of a loading property on google-map-search forced me to do a lot more event handling than
I would have liked. So, I added one.

Essentially, loading defaults to false. It is set to true in the search method, and it
is set to false once again in the _gotResults method. It is updated before google-map-search-results
is fired, so that event will properly occur once all search actions are complete.

I also added a unit test around searching. I noticed that most of the tests won't run properly
without an api-key set on the google-map, so I guess that we'll all just have to insert our
own during testing? Kinda strange.

Anyway, this will now let you easily notify your users that a search is in progress. Previously,
this had to be done with a lot more hooks and events.

<google-map-search map="[[map]]" results="{{results}}" loading="{{loading}}"></google-map-search>
<template is="dom-if" if="[[loading]]">
Loading...
</template>
<template is="dom-if" if="[[!loading]]">
<ul>
<template is="dom-repeat" items="[[results]]">
<li>[[item.formatted_address]]</li>
</template>
</ul>
</template>