Extra arguments escaping

There is a problem with unneeded extra escaping of helpers arguments.

If one want to pass string with a character that must be escaped to a helper he should use a code like the following:
```
{{someFakeHelper "It is a \"correct\" string."}}
```

But ```someFakeHelper``` cannot use its argument. Due to wrong escaping it will receive a string ```"It is a \\"correct\\" string."```. ```\Handlebars\Template::parseArguments``` method applied for the string returns the following arguments:
 - string ```It is a\```
 - variable ```correct\```
 - string ``` string.```

Thus there is no way to specify string arguments with characters escaping in a template.

I've attach a pull request with a test for the problem.