Improve python API: Add façade function to create python api sessions.

Add a utility function to make creating client session more comfortable.
For example, instead of:
```
from neovim import socket_session, Nvim
session = tcp_session(address=<address>, port=<port>)
nvim = Nvim.from_session(session)
```
you can do now:
```
from neovim import attach
nvim = attach('tcp', address=<address>, port=<port>)
```
Improve python API: Rename spawn --> child.
Improve python API: Fix PEP8 compliance.

- Two blank lines before function.
- Use def instead of assigning lambda to local variable.
Improve python API: Adapt tests.
Improve python API: Adapt README.