Change window and packet size to match opensshs'.

Update tests to match the new numbers.
Rename max_packet_size and window_size.

This is to indicate that they should be seen as defaults and can
be overriden.
Add possibility to set default sizes.

This change adds two new parameters to the constructor of the
Transport class. Letting the user set the default windows size and
default max packet size for the instance.
Support specyfiying window/packet size per session

Also update the documentation about values and the adverse effects
that changing them might create.
Expose the ability to set window/packet for sftp.
Whitespace fixes.
Bump the MIN_PACKET_SIZE to what's in the RFC.
Add a MAX_WINDOW_SIZE constant.
Add a utility method for value clamping.
Add sanitation methods for window and packet size.
Move window and packet constants.

Centralise them to the common module to avoid import cycles.
Also add constants for default values and use them in the
transport class.
Use the new packet sanitize method in channel.
Merge branch 'master' into 372-int

Conflicts:
	paramiko/channel.py
	tests/test_util.py
Add versionchanged notices, and some minor formatting tweaks
Changelog re #372