Moved SQLite database for demo app to SD Card.
Made demo app use transactions with database.

It could not handle saving full rate ISOBUS to the database before.
Improvements to `PGN` class.

* Class in now immutable
* Overrode `Object` methods to make comparisons work
* Fixed check(s) for invalid PGNs
Made PGN selector in demo app work.
Fixed crash when receiving invalid messages
Added overviews and usages examples

Put individual READMEs for libISOBlue and the demo app.
Added license header to files w/o one
Merge pull request #3 from awlayton/master

Usage documentation and demo app to log to SD card