Make the next release from master 3.0.0

[GrahamCampbell]

This is not technically necessary, however some might argue the change that has just gone into master is breaking. We could use this opportunity to also clean up some things from the code such as removing the deprecated authentication stuff, and changing out the HttpMethodsClient with HttpMethodsClientInterface.

There's no reason why a major release needs to mean big changes. Just some nice cleanup. ;)

[GrahamCampbell]

There's actually quite a bit of deprecated stuff accruing that could be removed.

[GrahamCampbell]

@acrobat I think the v3-dev branch can be deleted now, in favour of master.

[acrobat]

Yes indeed, there was no difference between those branches. v3-dev is removed!