Update to stop MBE's automatically display

Currently if we let g:miniBufExplBuffersNeeded=1|0, then open a file and do some modifies on it, MBE will automatically display, since we don't need it. So I made some changes:
If 'g:miniBufExplBuffersNeeded' OR 'g:miniBufExplBuffersNeeded' is less than 2, the MBE window will not auto open until the number of windows is greater than 1.
@see HasEligibleBuffers()