Make check space optional

This pull request has my last one (https://github.com/IMGIITRoorkee/django-filemanager/pull/1) wrapped up in it.  This one makes checking space restrictions optional with new settings.  For certain directory set ups (lots of files and/or large files), this can be unfeasible to do at start or at each upload.  We (for example) control space by other mechanisms, so it is redundant as well as time consuming.  A good feature to have, but also good to be able to disable.  The visual count can also be disabled since it no longer contains useful data.  

SB