Add option to authenticate using TransIP key-pair

[roaldnefs]

Is your feature request related to a problem? Please describe.
Currently authentication is implemented by allowing an access token to be used when initialising a new transip.TransIP client. As the access tokens have a limited validity, the user will have to create a new access token at least every month (the maximum expiration time).

Describe the solution you'd like
The TransIP API also allows the creation of an access token by using a private key. By allowing the user to specify either an access token or private key when initialising a new transip.TransIP client, the client could dynamically generate a new access token when a private key is used.

Additional context
Official API documentation about authentication: https://api.transip.nl/rest/docs.html#header-authentication.