Extended the Requirements section wrt installing the 'ocsync' binary.

Only installing that binary isn't enough, you also need to install the
owncloud plugin. This caused some issues, so hopefully no more.
Cleanup README
Update manpage for 0.3.1
using ERRNAMES instead of there value
Updated CHANGELOG
Added csync version to error output
Fixes: Authentication callback prints real password in debug mode

Close: #55