Window-edge detection

Detecting if there is enough space to render datepicker popup below the
field from its left edge - if not - changing popup position.
Positioning logic extracted to setPosition function
Code cleanup
Obtaining window dimensions in cross browser way
Merge remote branch 'achwedyk/adjusting_position' into adjusting_position
Picker top position is dynamically determined by viewport height.
Add #adjustPostion
Merge branch 'master' of github.com:dbushell/Pikaday
Fix #8 and close #45
Add speed improvement by using getBoundingClientRect when available in #adjustPosition
The element’s left and top values are already correct before this adjustment