-initWithBaseURL: will now raise an exception with a nil URL parameter. This enforces what the code comments say.

Without this, Reachability might cause the application to crash on iOS 6 if the baseURL is nil.
[Issue #548] Reverting breaking changes from b317c7d5d9e23f2516b6b917249eb4705cab74e4
Merge pull request #547 from jbrennan/bugfix/httpclient-nil-url-exception

-[AFHTTPClient initWithBaseURL:] will now raise an exception with a nil URL parameter.
[Issue #554] Re-adding AFMultipartFormData -appendPartWithHeaders:body:
[Issue #555] Leaving square brackets unescaped in query string parameters
Merge branch 'master' of github.com:AFNetworking/AFNetworking

Conflicts:
	AFNetworking/AFHTTPClient.m
Removing outdated note about 3rd party JSON libraries
Updating documentation for multipart form request method
Fix type discrepancy for upload stream delay.

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>
Merge pull request #558 from rastersize/fix-upload-stream-delay-type

Fix type discrepancy for upload stream delay.
[Issue #559] Fixed body for NSNull parameter values in multipart/form-data requests

Parts for parameters with a value of NSNull now have an empty body rather than "<null>" (which was produced by -[NSNull description]).
Merge pull request #559 from danielr/fix-nsnull-in-multipart

NSNull parameters are encoded as "<null>" when using multipart form data
Use isEqual: to determine if value is NSNull
If a query string pair has a nil or NSNull value, just use the field as the string value
[Issue #557] Replacing AF_DISPATCH_RETAIN_RELEASE macro with OS_OBJECT_USE_OBJC (thanks, @nakiwo)
Minor Reformatting
Enforcing 'this parameter must not be nil' by using NSCParameterAssert()
Adding #warning macros when SystemConfiguration or CoreServices frameworks are missing, causing functionality to be missing or altered
Fixing OS_OBJECT_USE_OBJC macro check
Using better negation for OS_OBJECT_USE_OBJC macros