Adds Boto profile argument for multiple credential support.

Reference:
http://boto.readthedocs.org/en/latest/boto_config_tut.html#credentials

Usage:
```
ec2yaml --boto-profile Personal test.yaml
```

Where "Personal" is defined in `.boto` as:
```
[profile Personal]
aws_access_key_id = <access key for this profile>
aws_secret_access_key = <secret key for this profile>
```