Initial refactoring. Added some new features

- renamed package to it.fabiolbr.nfctagwriter
- added icon
- adjusted the layout(margins, small fixes, better on tablets)
- changed initial text of views
- changed text of some toasts
- check if MIME & Value are null (visual error integrated into them)
- extracted hardcoded string
- added private instance of views\listener
- added check if nfc is enabled or not
- simplified onClick method(I'll probably remove the button anyway)
- autowrite: no need to tap the write button
- requirements: min sdk=16,  nfc=true