Support a user-specified SSH username.

This commit enables the user to specify a SSH username for the
provider using the ssh_username config attribute. For the initial
droplet creation and provisioning, the provider will fallback to the
root user. On subsequent execution of Vagrant commands, the
specified SSH username will be attempted instead. The user should
create the SSH username account during the provisioning process.

Additionally, better state checking for a droplet was implemented.
If a droplet is already active, calling up will have no effect. The
same is true when calling destroy, provision, or rebuild after a
droplet has been destroyed.

Closes #23 and #25.
Merge pull request #26 from smdahlen/ssh-user

Support a user-specified SSH username.