Fix missing else braces from @JelloRanger's changes

- Add missing braces for else block and fix tabbing to properly execute
necessary methods for each condition
- Add comments to explain if block statements
Allow app usage without NFC enabled

- Revise requireNfcEnabled() to allow for use of the app with devices
that have NFC hardware, but have the adapter disabled currently only if
they have not selected a file as if they are going to send it
- Revise "How to Use" paragraph for conciseness and accordance with
Android message guidelines
Merge pull request #35 from pdpiech/master

Fix @JelloRanger's changes and revise SendFragment to allow NFC to be disabled