Add weather background, fix bugs, secure code

Add beautiful picture for a WeatherFragment background
Change font sizes to be specified in 'sp' in all layouts for
cross-device compatibility (instead of fixed 'pt' sizes)
Secure all getters of classes in edu.rpi.rpimobile.models to return
copies of objects
Fix various duplication bugs
Fix various refresh button bugs
Change WeatherHttpClient class to more generic HttpClient class
Remove ProGuard optimization configuration files
Remove unused 'prefs_twitter.xml' file
Clean code formatting