bug fix: read css file name instread of content in cre:setStyleSheet
add setStyleSheet method to credocument
add default_css to credocument
add readertypeset to readerui
add readertypeset.lua
Merge branch 'new_ui_code' of github.com:hwhw/kindlepdfviewer into new_ui_houqp
API changes in cre.cpp

* now css argument is removed from newDocView call.
* when setStyleSheet method cannot read given css file, it
  clears all the applied style.
* add setEmbeddedStyleSheet method.
adapt credocument to new changes in cre.cpp
two changes to ReaderTypeset

* now we only use Embedded CSS by default
* add toggleEmbeddedStyleSheet methods
cosmetic changes to menu wording
set self.css to nil when there is no external css
Merge pull request #721 from houqp/new_ui_houqp

add css file selection menu for crengine