Make all ginclient functions methods of a client struct

The ginclient package should have a struct that holds a configuration and implements methods for all the functionality it supports.

As is conventional with Go packages, it could also have top-level equivalent functions that use a default client.

Subtasks:

[ ] Client instance should handle config read/write
[ ] Client instance should handle logging
[ ] Configuration values for web and git (address, port) are passed as arguments to the New... function.
[ ] Imports git for git and annex commands.
[ ] Imports web for web calls/requests.

G-Node / gin-cli