Added Blockchain Bootstraping Tutorial

Per @laanwj request in #3724

Fixing Checkpoints Comment

Fixed Image Paths

Removed requirement.

Update Torrent Links.

Update Links 2

Link would not work.

2 Small Fixes

Changed in to from

@laanwj suggestion on safety
Merge pull request #3747

b698417 Added Blockchain Bootstraping Tutorial (super3)
qt: remove de_AT translation

Got too many complaints that is was unserious and written by trolls.
I have also removed the translation from transifex.
Fixes #4054 and #3918.
[Qt] catch Windows shutdown events while client is running

- prevents unsafe shutdowns on Windows, which is known to be
  able to cause problems with wallet.dat
- if a users ends a Windows session, this will initiate a client shutdown
  and show a Windows dialog, that tells the user what is going on (for
  Windows Vista and higher it will even show a reason for blocking the
  Windows session end)