Add 'go to page' feature.
- Hash of URL indicate current slide index, and when hash of URL changed then show the slide which specified by hash.
- 'Slide n of m' change from static text to drop down list.
- PowerPoint compatible keyboard shortcut: press some numeric key and enter, for example, '2','1',[Enter], then show the slide page 21.
Merge pull request #9 from jsakamoto/feature/goto_page

Add 'go to page' feature.