Prepared gitignore for CocoaPods and workspaces
Created a SDWebImage workspace with the 2 existing projects, fixed the demo project so it directly references the source files. Shared a demo project scheme
Added SDWebImage Tests project. It uses CocoaPods (Expecta + XCTestAsync). Added some demo tests
Added error test
Reverted demo project referencing source files directly. Re-added SDWebImage project as subproject
Merge branch 'unit_tests'

Unit testing setup + created workspace
- created a SDWebImage.xcworkspace
- includes the existing SDWebImage.xcodeproj (responsible of compiling the library)
- includes the existing SDWebImage Demo.xcodeproj
- includes the newly created SDWebImage Tests.xcodeproj (tests project). The test project uses the local CocoaPods repo to link to the library

Unit testing backbone:
- XCTest framework (the one from Apple)
- XCTestAsync - extension for XCTest that allows easily creating asynchronous tests. Relies on dispatch_after.
- Expecta - matcher Framework for Objective-C/Cocoa

Updated gitignore