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


The DO implementation will erase the custom docker (with identity auth) with the one from the distrib whenever the distrib upgrades its docker. So, this doesn't seem the right thing to do. If there is an URL where to get an updated version (like the one we use to get the docker executable), we can do that instead.