No leniency for numerics in matchers


Added ability to extend `be_close_to` like `equals`.

**Breaking change** - This removed the implicit conversions of the following types between each other:

- C numerics
- NSNumber
- NSDecimal
- NSDecimalNumber

This was the cause for bugs like:

```
(NSNumber *)@"5" should equal(@5);
(NSNumber *)@"5" should equal(5);
```

to pass.