Separate Flask server from Cubes

[Stiivi]

Separate the flask slicer serve functionality from cubes.server into a separate module.

Reasons:

• allow other server type implementations, such as protobuffer or thrift kind of server.
• make the core Cubes package more light weight
• decrease dependency complexity
• open possibility for separate maintainer

Changes needed:

• have a server extension type
• define server extension API
• add server loading mechanism
• add type into [server] section of slicer.ini

Recommended external server module naming: cubes-server-TYPE such as cubes-server-FLASK for stand-alone packages with server implementations. It is just a convenience, since we are going to use entry_points anyway.

Default server type should remain flask and will not have to be specified explicitly in the slicer.ini.