Convert all singleton global variables to sub-classing configuration of the application.

[puncsky]

For global variables, subclassing instead of singleton.

This is a following-up issue from #28

http://stackoverflow.com/questions/708012/how-to-declare-global-variables-in-android

Cons of singleton

• Inability to use abstract or interface classes;
• Inability to subclass;
• High coupling across the application (difficult to modify);
• Difficult to test (can't fake/mock in unit tests);
• Difficult to parallelize in the case of mutable state (requires extensive locking);
• Unclear and unmanageable lifetime contract unsuited for Android (or most other) development;