title: Raise an NSDecimalNumber to a negative power
body: <p>I need an equivalent to C's pow() function that will work with NSDecimalNumbers. With pow you can use negative numbers e.g. <code>pow(1514,-1234)</code>, with NSDecimal's decimalNumberByRaisingToPower: method, you are forced to use an NSUInteger which seems to require a positive value.</p>
  
  <p>I'd like to be able to do something like this:</p>
  
  <pre><code>[decimalNumber decimalNumberByRaisingToPower:-217.089218];
  </code></pre>
  
  <p>or</p>
  
  <pre><code>[decimalNumber decimalNumberByMultiplyingByPowerOf10:-217];
  </code></pre>
  
  <p>without crashing from overflow or underflow exceptions.</p>