support for regions

[jantman]

Some of the boto services (i.e. RDS) don't have a region-setting ~/.boto option, and the only way to set the connection region for them is in whatever is using boto (i.e. this project). We need to support setting regions.

For the time being, we're not adding multi-region support to awslimitchecker, we're just adding support to set regions other than boto's hard-coded us-east-1 default. We'll need to pass this to the individual service connectors, TA, and anything else that connects to AWS.

Possible options:

1. Make this a command-line option, and also have a way to set it on the main Checker class from Python.
2. In all of our service connectors, simply check for an AWS_REGION environment variable, and pass it in to the connector if it's specified.

[jantman]

This is already part of #64; I just need to get that cleaned up and merged.