Changed method of activating preferred exchange.

Instead of using integer index offsets for storing the preferred exchange tab, each tab is now "tagged" with the exchange and stored as a preference item. This will allow future enhancements to allow specifying the visibility and tab re-ordering.

Also includes code to migrate from the old integer index method to the new tab tag method.
Fixed default value for preferred exchange.
Merge remote-tracking branch 'upstream/develop' into develop

Conflicts:
	res/values/exchanges.xml
	src/com/veken0m/bitcoinium/MainActivity.java
Merge remote-tracking branch 'upstream/develop' into develop

Conflicts:
	res/values/exchanges.xml
	src/com/veken0m/bitcoinium/MainActivity.java
Changed tab keys from integer indexes to strings
Replace index-based ID with string-based ID

Changed usage of index-based IDs in widget and Exchange class with string-based IDs used elsewhere.
Merge pull request #15 from zerodivide1/develop

Develop