Allow spaces within media query features

This (note whitespace around `max-width: 710px`) should be valid:
```css
@media screen and ( max-width: 710px ) {}
```

Currently the whole line errors out:

```
[L1:C20]
>> ERROR: Expected IDENT at line 1, col 20. This rule looks for recoverable syntax errors. (errors) Browsers: All
[L1:C32]
>> ERROR: Expected LBRACE at line 1, col 32. This rule looks for recoverable syntax errors. (errors) Browsers: All
[L1:C32]
>> ERROR: Unexpected token '710px' at line 1, col 32. This rule looks for recoverable syntax errors. (errors) Browsers: All
[L1:C38]
>> ERROR: Unexpected token ')' at line 1, col 38. This rule looks for recoverable syntax errors. (errors) Browsers: All
[L1:C40]
>> ERROR: Unexpected token '{' at line 1, col 40. This rule looks for recoverable syntax errors. (errors) Browsers: All
[L1:C41]
>> ERROR: Unexpected token '}' at line 1, col 41. This rule looks for recoverable syntax errors. (errors) Browsers: All
```

This PR fixes CSSLint/CSSLint#541 with a simple `this._readWhitespace();` call, and includes tests.