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
alternating row colors and layout tweaks
use https for Bitcoinium.com and Bitcoinity
update XChange and resCU jars
Release the Kraken!
fix 4 digits display for bid/ask on widget
Eligius mining pool
Widget tap now selects Exchange or Pool tab
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
Update README.md
improve Activity lifecycle management
if the main view is "null" don't look for table
Add LTC/EUR, NMC/USD and NVC/USD pairs for BTC-e
Merge branch 'develop' of https://github.com/veken0m/bitcoinium into develop
cleanup and improve exchange identifier

- "prefix" and the new exchange "identifier" that replace integer-based
identification serve the same purpose, so they were merged