Introduce ClosingContextManager for classes that already implement close()
Convert SFTPClient to a context manager
Convert Channel into a context manager
Convert Transport to context manager
Fix error on Python 2.5
Remove TODO
Document that classes can be used as context managers
Turn ProxyCommand into a context manager
Turn BufferedFile into a context manager
Turn SFTPHandle into context managers
Convert SSHClient into a context manager
Merge branch 'master' into 216-int

Conflicts:
	paramiko/transport.py
	paramiko/util.py
	tests/test_client.py
Merge branch 'master' into 216-int
Clean up & tweak failing sftp closure test
Merge branch 'master' into 216-int
A herp and a derp (and an import shuffle)