Added SDNetworkActivityIndicator library and integrated it in plugin.xml definition as well as CordovaHttpPlugin.m

This library allows to show the network activity indicator on iOS devices, to indicate native networking
It uses a counter to hide the indicator depending on whether all request have been fulfilled or not

src/ios/SDNetworkActivityIndicator/README.md

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+# SDNetworkActivityIndicator
+
+Handle showing / hiding of the iOS network activity indicator to allow multiple concurrent threads to show / hide the indicator such that the indicator remains visible until all the requests have completed and requested the indicator to be hidden.
+
+## Requirements
+
+* iOS 5.0 or later.
+* ARC memory management.
+
+## Installation
+
+The easiest way to install it is by copying the following files to your project:
+
+* SDNetworkActivityIndicator.h
+* SDNetworkActivityIndicator.m
+
+## Usage
+
+* When you start a network activity (will show the network activity indicator):
+
+        [[SDNetworkActivityIndicator sharedActivityIndicator] startActivity];
+
+* When you finish a network activity (will hide the network activity indicator only if the number of calls to `stopActivity` matches the number of calls to `startActivity`):
+
+        [[SDNetworkActivityIndicator sharedActivityIndicator] stopActivity];
+
+* To hide the network activity indicator regardless of whether all activities have finished (without having to call `stopActivity` for each `startActivity` called):
+
+        [[SDNetworkActivityIndicator sharedActivityIndicator] stopAllActivity];
+
+
+## License
+Copyright (c) 2010 Olivier Poitrey <rs@dailymotion.com>
+ 
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is furnished
+to do so, subject to the following conditions:
+ 
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+ 
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.
+