New camelize filter #77

The test are amazing, and thanks to write that. But the behavior you wrote is not the correct one for the first letter. 
Here there are the test I would like replicate 
https://github.com/epeli/underscore.string/blob/244b1724770947430793dc43fb96d610dd045c47/test/strings.js#L388
and as you can see 
```
equal(_('the_camelize_string_method').camelize(), 'theCamelizeStringMethod');
```
instead in your case you wrote
```
expect(camelize('test_string')).toBe('TestString');
```

In javascript when you write a variable name in camelCase the first letter should be in lower case and not uppercase.