Switching to NSMutableString for performance improvements

As discussed in issue #3, switching to `NSMutableString` gives a pretty nice performance boost. I've also added a convenience method to `NSString` (using a category) which simply returns a mutable copy of the string, but explicitly stating the return type to be `NSMutableString` instead of `id`. This fixes problem when user tries to write `@"".mutableCopy.a.b.c`. 

**WARNING:** This PR breaks compatibility with older versions, and will cause compile errors when a user tries to compile old code with new version of this category.

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So, the new syntax looks like this:

```
@"".mutableString.a.b.c.d
> @"abcd"
```