title: Validating alphabetic only string in javascript
body: <p>How can I quickly validate if a string is alphabetic only, e.g</p>
  
  <pre><code>var str = "!";
  alert(isLetter(str)); // false
  
  var str = "a";
  alert(isLetter(str)); // true
  </code></pre>
  
  <p>Edit : I would like to add parenthesis i.e <code>()</code> to an exception, so </p>
  
  <pre><code>var str = "(";
  </code></pre>
  
  <p>or </p>
  
  <pre><code>var str = ")";
  </code></pre>
  
  <p>should also return true.</p>