Adding ec2_elb_facts module

This module fetches data from AWS using the boto api and returns
a hash as a fact named ec2_elbs that maps instances to the ELB(s)
that they belong to.

example:
```
     - name: Collect elb facts
       local_action: ec2_elb_facts
```

```
$ ./test-module -m ../library/ec2_elb_facts 
{"changed": false, "ansible_facts": {"ec2_elbs": {"i-12345": ["elb1", "elb2"...], ...}}}

```