background color and pause feature

set background to black and play puase using spacebar
Added distribution term and made window active when launched

added the code distribution termis under BSD Licence and updated code
to make window active when launched and edited docsting of Keyevent
method.
Merge pull request #1 from inhumanitas/master

Made code review. a little pep8
optimized to PEP8 and refactored
used QMainWindow instead of QWidget and removed the requirement of QHboxLayout and made public variables protected made common function of supported image formats.
refactored image list passed of only supported image formats