fix a minor issue in README.md

And also other minor changes
Fix an issue which creates 256 empty entries in 'content.opf'
fix the issue that cover thumbnail is not visible

change the way to handle cover page.
before: user create 'cover.html' as the cover page
now: use 'cover.png/jpg/gif' as cover image and generate cover page
'cover.html' automatically
update the example files
Add the option of chapter separator

if separator is 'header', will split chapters at <h1>, <h2>, ...
for example:
	<h1>chapter 1</h1>

if separator is 'comment', will split chapters at the commented <h1>,
<h2>, ...
for example:
	<!--<h1>chapter 1</h1>-->
remove UTF8 BOM and improve performance

Detect and remove UTF8 BOM, this makes it is easy to handle windows
NOTEPAD edited files

Improve performance by replace 'scanner.Text()' with 'scanner.Bytes()'.
Fix bug: files in sub-folders cannot be accessed
Completely fix the bug: files in sub-folders cannot be accessed
updated for EPUB3 support, not completed yet