driver: exoscale driver

Add support for exoscale, a Swiss cloud provider. This pull [egoscale](https://github.com/pyr/egoscale),
a Go binding for exoscale, in godeps which has no additional dependencies.

## Configuration

The main configuration options are:

- `--exoscale-api-key`: exoscale API key, also looked in the `EXOSCALE_API_KEY` environment variable
- `--exoscale-api-secret-key`: exoscale API secret, also looked in the `EXOSCALE_API_SECRET` environment variable
- `--exoscale-instance-profile`: name of exoscale profile to use, defaults to `small` and must be one of: `tiny`, `micro`, `small`, `medium`, `large`, `extra-large`, `huge`
- `--exoscale-disk-size`: disk size to use, in GB. one of: 10, 50, 200, 400

## Default Behavior
By default, an SSH keypair will be created for each new machine and a security group will be created authorizing TCP ports 22 and 2376 as well as ICMP echo.

The `--exoscale-keypair` and `--exoscale-security-group` options may be used to modify this behavior.
The default image used is a 64-bit Ubuntu 14.04 image and may be changed with the `--exoscale-image` option.
The only valid image types to use for now are: "ubuntu-14.04" and "ubuntu-14.10"

## Docker machine support

The following docker machine commands work as expected:

- `ls`
- `ssh`
- `start`, `stop`, `restart`, `kill` and `rm`
- `create`
- `inspect`

The only command which provides no behavior is the `upgrade` command.

## Testing out

Testers wishing to validate docker-machine's behavior on exoscale may use the following coupon code (first come, first served): DOCKERMACHINEROCKS on https://portal.exoscale.ch


Perhaps we should have drivers implement driver, but add a warning in the pre-release version of Machine that upgrading doesn't work yet.