Added support for auth_method in push() and authenticate()

[ivanharvard]

Closes #54.

Now you can write:

from lib50 import authenticate

with authenticate("me50", auth_method="https") as user:
    print(user.name)

or

from lib50 import authenticate

with authenticate("me50", auth_method="ssh") as user:
    print(user.name)

to force a specific authentication method, instead of trying SSH first and then trying HTTPS once SSH failed.

This argument also exists in push.