Support storing the token in an encypted form

[alahiff]

Linux, Mac and Windows all have default ways of storing credentials in a way which is more secure than plain text. We should provide the ability to make use of this easily.

[kzscisoft]

Implementing this is quite easy, for now I have it so that if the user specifies a URL in their config but no token it will try to retrieve it using keyring.get_password.

However some points for thought:

• The token/password is stored under a service + username combination. At the moment I get use the URL as the service (fine) and the current login username as the user. It doesn't actually matter what the username is as long as its the same username used to create the key.
• The question is what instructions do we give a user to set this up? Do we tell them to use their Simvue user (the issue here is that they then have to provide this in the config also), do we tell them to use their login user (fine), or do we have something within the python interface which sets this up for them?