Log Synnefo management commands to files

Create 'SynnefoOutputWrapper' which replaces Django's 'OutputWrapper'
and logs the command and its output in a file.

Ref #3: Log all stdout/stderr for snf-manage invocations
Update Changelog about commands logging mechanism

Add an entry to the Changelog about the 'Logging mechanism for Synnefo
management commands'.

Closes #3: Log all stdout/stderr for snf-manage invocations