Update README.md for 'complete documentation' link

http://afnetworking.github.com/AFNetworking is no more.
Fixing -[AFHTTPClient copyWithZone:].
Put the logo back

It's prettier that way.
Merge pull request #1116 from Palleas/patch-1

Put the logo back
Merge pull request #1114 from OliverLetterer/copy-http-client

Fixing -[AFHTTPClient copyWithZone:].
Merge pull request #1113 from raven/master

Update README.md for 'complete documentation' link
[Issue #1115] Only set placeholder image if not nil
Merge branch 'master' of github.com:AFNetworking/AFNetworking
Merge remote-tracking branch 'AFNetworking/master'

* AFNetworking/master:
  [Issue #1115] Only set placeholder image if not nil
  Put the logo back
  Fixing -[AFHTTPClient copyWithZone:].
  Update README.md for 'complete documentation' link