Button code, bugfixes,(Read please!)

- Removed what is now considered toxic code (PLEASE look into your .git
folder for a file called MERGE_HEAD and DELETE IT).
- Added variables to hold strings from resource that are used for
buttonClick.
- Added method initializeSigns in onCreate to attach resource strings to
the respective variables.
- Simplified switch code by using the resource string variables and by
removing the 'else' statement from between the textString and textInput
statements.
- Fixed a previous issue where the mathematical signs were not being
displayed when their buttons were pressed.
- Fixed an issue where the plus sign was being printed when any
mathematical sign was pressed.
- Fixed a potential issue where copies of one sign would be printed.
- Fixed an issue where the app would crash if a mathematical sign is
pressed when the TextView is empty.
- Small edit in the dimens resource to slightly shorten the switch (+/-)
sign in the button.