Prevent nil inseration exception on failure without callsite

If a KWFailure was initialized without a callSite an exception would be
raised while trying to genrate the exceptionValue for trying to insert nil
into the user info dictionary.
Merge pull request #221 from drewag/master

Fix the hiding of true failure reason if no callSite is set on KWFailure
Receive matcher now stubs its expected message, as in RSpec.

It does not override an existing stub, if you have manually stubbed the message before.
Removed KWMock captureArgument:atIndex method since it has been moved to NSObject level
Added class-level stubbing and spying methods.

The functionality was probably there, but Xcode did not provide autocomplete and it is not right to use instance methods on the class level.
Changed the suggested installation method to CocoaPods
Merge pull request #226 from allending/cocoapods_readme

Changed the suggested installation method to CocoaPods
Merge pull request #224 from stepanhruda/class-method-spies

Added class-level stubbing and spying methods
Fix spelling error
Merge pull request #228 from thoughtadvances/master

FIx readme.me spelling error
Merge pull request #223 from stepanhruda/receive-stubbing

Receive matcher now stubs its expected message, as in RSpec.
Compile issue with pull request 223
Merge pull request #230 from timle8n1/master

Fix compile issues in pull request 223
Added support for partial mocks
Merge pull request #231 from danielrhammond/partial-mocks

Added support for partial mocks
fix #185 - dictionary failures now have keys and values
Merge branch 'master' of https://github.com/allending/Kiwi into convert-to-arc

Conflicts:
	Classes/KWFormatter.m
	Classes/KWMock.h
	Classes/KWMock.m
	Kiwi.xcodeproj/project.pbxproj