Multi size support

Hey, thanks for the great widget; anumodana :)

Hope this is somehow helpful, here are the changes in brief:

- Upped targetSdk to 17 (this is recommended by the SDK)
- added `supports-screens` tag, which allows the WebView to show correctly on larger screens - I don't think there are any screen sizes this app shouldn't support...
- simplified main.xml layout - WebViews are already scrollable, so there shouldn't be need to put one in a ScrollView; also forced to fill screen
- added zoom controls & pinch-to-zoom capability (if present) to WebView - on tablets this is useful since the text is fairly small.