add reading position on each page
Page position is a fractional number ranging from 0 to 1, indicating the read percentage on
certain page. With the position information on each page whenever users change font size,
page margin or line spacing or close and reopen the book, the page view will be roughly the same.
enlarge OCR region
try to drop cache before OCR
comment revision
Merge pull request #88 from chrox/reading_pos

save/restore reading position on each page