add methods to retrieve elb tags

example:
```
import boto.ec2.elb
celb = boto.ec2.elb.connect_to_region('us-east-1', aws_access_key_id='...', aws_secret_access_key='...')

elbs = celb.get_all_load_balancers()
celb.get_all_tags([e.name for e in elbs])
for elb in elbs:
    print elb.get_tags()
```