Add KWContainStringMatcher

Adds new matcher class, KWContainStringMatcher, to support expectations `containString:`, `containString:options:`, `startWithString:`, and `endWithString:`.

Example usage:

     [[@"Hello, world!" should] containString:@"world"];
     [[@"Hello, world!" shouldNot] containString:@"xyzzy"];
 
     [[@"Hello, world!" should] containString:@"WORLD" options:NSCaseInsensitiveSearch];
 
     [[@"Hello, world!" should] startWithString:@"Hello,"];
     [[@"Hello, world!" should] endWithString:@"world!"];

For more details and examples, see https://gist.github.com/kristopherjohnson/5478039