Allow SSH class to be used with 'with' statements

[Jesse Almanrode]

Add the enter and exit functions to the SSH class to allow it to be used with Python's 'with' statement.

I totally want this:

with SSH('localhost', username='jdoe', password='jdoe') as s:
    s.ssh_command('uptime')

[Jesse Almanrode]

Added in commit f54ac3d

Will be published in v3.0.1

[Jesse Almanrode]

Removing version: v3.0 (automated comment)