WIP: Open Street Map Loader

This is work in progress, but I wanted to post it for discussion.

The goal here is to be able to load road data from OSM such that it can be used to create a Pandana network.
Here's an example of the type of query I have coded in: http://overpass-turbo.eu/s/6AK. It's a "way" query for a bounding box with "recursion" so that it goes and finds all the nodes that are attached to the found ways. The actual query looks like:

```
[out:json];
(
  way(37.8659303546,-122.2588003879,37.8661598571,-122.2585062512);
  >;
);
out;
```

The JSON result of that query can be turned into three DataFrames: nodes, ways, and waynodes.

The nodes table has a schema like:

```
id    lat    lon    tag1    tag2 ...
```

The ways table has a schema like:

```
id tag1 tag2 ...
```

And the waynodes table has a schema like:

```
way_id    node_id
```

In each of those the first column is really the index.

@fscottfoti, is that the kind of information we need, or is there more? We could also filter this to certain tags, like `["highway"="*"]` so we only get road ways (I think, my understanding of OSM data is still developing).