Refactoring and minor improvements

Made the following changes to htmlbook-xsl code base:

* Refactored indexgen.xsl so that index-letter normalization translate() values are encapsulated in variables, so the code is much cleaner and easier to read
* indexgen.xsl now generates index using \<ul\> markup in accordance with the latest EPUB 3 Index Specification Working Draft
* Tweaked handling in chunk.xsl so that target filename only gets prepended to href value when source link and target element are in different chunks