Ignore Thumbs.db (windows meta-data file)
Fix ValueError on remove_background()
When selecting a range by remove_background(), a ValueError is
raised initially when the left value is assigned, and the right value is
yet to be assigned (since the left can be higher than the right value).
Avoided by:
1) Setting default values to NaN, and not update line in plot until both right and left are set.
2) Checking which value is truly the left and right one before drawing line.
Small fix to remove_background() change
Moved range selector left-right switch to widget

Previously suggested switching left and right in
BackgroundRemoval.bg_to_plot(). Here changed to doing it in
ModifiableSpanSelector, by changing which edge is moved when crossing
over.
Made ModifiableSpanSelector edge switch strict

Fixed imprecision in edge switch in ModifiableSpanSelector when the
change in position was large, i.e. it now precisely keeps the position
of the edge being passed over.
Revert ModifiableSpanSelector edge crossing
Merge pull request #260 from vidartf/master

 Fix ValueError on remove_background() and ignore Thumbs.db