Significantly improved XML data validation

Prototyping: * Set DEV_MODE if the app isn't signed (not in market)
* Added tons of XML validation:
+ Check if specified image exists
+ Check if values are valid ints without crashing
+ Warn if image attrib exists, but is empty
+ On-screen log shows what book was loaded and how many valid pages
+ Check for duplicate page IDs
+ Check for invalid destination IDs (unwritten pages)
Loads XML from SDCard now

And handling missing file