Button functions, TextView, code tune-up

- Removed the EditText field in favor of a TextView form
- Added a context to obtain data from the project's resources and the
project itself
- Added a StringBuilder to store raw user input.
- Renamed "buttonClickHandler" to simply "buttonClick"
- Added several string characters to the strings.xml resource
- Modified the switch statement to use the StringBuilder and TextView
- Fixed an issue where the symbols buttons (+, -, /, etc.) did not
function
- Added functionality to all the buttons except the equals and inverse
buttons
- Removed extra lines of code that would have otherwise caused problems
to the project
- Changed the calculator layout so that the "DEL" button is next to the
expand button and the inverse button fits better in the display