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.