adds support for config file

Fixes #46

I've taken a look at issue #46 for adding a config file to denominator.

Here's the syntax we've been kicking around on IRC:

	./denominator -C /path/to/config.yml -n route53-test zone

For example, you could configure both route53 and ultradns like above then using only -n arg, run zone list.

The proposed config file format is a list of providers and relative credentials:
```
name: route53-test
provider: route53
credentials:
  accessKey: foo1
  secretKey: foo2
---
name: ultradns-prod
provider: ultradns
credentials:
  username: your_username
  password: your_password
```