Avoid "No handlers could be found for logger".

Some processes such as zenjobs haven't yet configured the root logger
when zenpacklib initializes. This results in the following message
potentially being printed:

	No handlers could be found for logger "zen.zenpacklib"

This update avoids that message by adding a NullHandler to the
zen.zenpacklib logger.
Log impacts/impacted_by definition errors.

Previously you would not be alerted if you made a typo in a class'
'impacts' or 'impacted_by' definition. Now you will see a WARNING log
such as the following:

    no '<name>' relationship or method for '<class>'
Warn when component grids are made too wide.

Assuming a design criteria of wanting component grid panels not to
horizontally scroll on a 1280x window, you cannot exceed 750 pixels
worth of custom columns. This update emits a WARNING log such as the
following if you exceed 750 pixels of custom columns on a class.

    <zenpack>: <class> custom columns exceed 750 pixels (<pixels>)