Tried to fix the strange NullPointerException.

This exception occoured when the path to the key-files directory
is null... wich it should never be! (CreateKeyMapActivity)
Merge branch 'master' of github.com:ikarus23/MifareClassicTool
New release: 1.7.1

Updated changelog.
Increased version number.
Added menu for preferences, etc. & about dialog.
Renamed new menu & some typo fixes.
Added bare bone of "Preferences" view.
Added GUI elements for preferences view.
Some layout fixes for the write tag options.
Preferences GUI is now fully implemented.

But until now the preferences will take no effect.
Added link to F-Droid store.
Added a enumeration for all preferences.
Refactored "Pref" to "Preference".
Fixed wrong infos on the auto reconnect prefernece.
Implemented the auto reconnect preference.
Merge branch 'preferences'
Refactored <name>Activity --> <name>.

Refactored MainActivity --> MainMenu.
Refactored HelpActivity --> HelpAndInfo.
Added tools:context to all activity layout.
Changed "menu_" strings to "action_" strings.
Changed "OK"-button to "Save"-button in save dump dialog.
Added "rate this app" button in about dialog.
Added link to changelog (about dialog).