Added Options Page

Added Options Page allowing user to customize shortcuts and other
parameters
Options Page adjustments

Implemented the adjustments suggested by Ilya: default values
adjustments, input validation, UI
Added "Remember Playback Speed" option

Option to control the use of the last video's playback speed for a newly
loaded video
Minor adjustments

Changed the default value for Remember Playback Speed to 'false'
Fixed negative playback speed on rewind
Merge pull request #7 from RaduFi/master

Added Options Page