Changes Log:

Improvements:
1. Added 'fit' option so it is possible now to fit background image into
browser viewport
2. Added callback calling into resize.backstretch event
3. Image's object is now accessible inside callback via 'this' pointer

Fixes:
1. If browser supports fixed position try to use the window's innerWidth
first like used for height before
2. Ommited unnecessary useWindowInnerHeight variable
3. Correct calculating of backgroung dimension for horisontal and
vertical viewport orientation
4. Correct centering X or Y, it is not related now from image ratio
5. Replaced double call of image css setting in _adjustBG with one by
extending bgCSS properties

# Conflicts:
#	jquery.backstretch.js

…create detailed example and expand readme.md with new option 'fade'