Make access/secret properties optional for S3 cache, add path property 

Boto doesn't require access/secret to create a S3 connection, Tilestache shouldnt either.  In absence of passed arguments Boto will look for ENV variables

```
AWS_ACCESS_KEY_ID
AWS_SECRET_ACCESS_KEY
```
It will also use instance IAM credentials (access/key/token).  I can't find any official documentation on this behavior besides this stackexchange answer from the dude that wrote Boto (http://stackoverflow.com/a/11130701/168256)  but I can verify it works :)

Also added an optional path property for specifying a path in the bucket to put tiles