Compile AFNetworking for iOS 6 and iOS 7

- By using __IPHONE_OS_VERSION_MAX_ALLOWED and
  __MAC_OS_X_VERSION_MAX_ALLOWED, it will allow users who have minimum
  of iOS 6 or Mac OSX 10.8 to levarage NSURLSession for users on iOS 7
  and Mac OSX 10.9
Compile AFNetworking for iOS 6 and iOS 7

- By using __IPHONE_OS_VERSION_MAX_ALLOWED and
  __MAC_OS_X_VERSION_MAX_ALLOWED, it will allow users who have minimum
  of iOS 6 or Mac OSX 10.8 to levarage NSURLSession for users on iOS 7
  and Mac OSX 10.9
Remove the lock podfile

It doesn't matter what version pods are for testing, this isn't a production
app. It's more cumbersome to ensure the lockfile is in-sync than the benefits
it's worth.
Fix README typo
Get responseString in correct encoding
Get encoding from charset and fallback to property
fix to setBackgroundImageForState works correctly. It causes response serializer is set to af_imageRequestOperation instead of af_backgroundImageRequestOperation.
Merge pull request #1481 from dataich/master

fix to setBackgroundImageForState works correctly.
Merge pull request #1480 from xHunter/master

Correct encoding for AFJSONResponseSerializer
Minor reformatting
Merge pull request #1445 from kylef/podlockfile

Remove the lock podfile
Adding Tests/Podfile.lock to .gitignore
Merge pull request #1470 from chellmuth/patch-1

Fix README typo
[Issue #1431] Use default authentication handling unless a credential exists for the server trust
Merge branch 'master' of github.com:AFNetworking/AFNetworking
Adding responseObject property to AFHTTPRequestOperation
[Issue #1388] Fixing regression in nested multipart parameters
[Issue #1471] Start network reachability monitoring automatically for request operation and session managers
Minor formatting
[Issue #1475] Adding missing text/javascript MIME type from JSON response serializer documentation

Signed-off-by: Mattt Thompson <m@mattt.me>