title: Select subset of rows of dataframe using multiple conditions
body: <p>I would like to select a subset of a dataframe that satisfies multiple conditions on multiple rows.  I know I could this sequentially -- first selecting the subset that matches the first condition, then the portion of those that match the second, etc, but it seems like it should be able to be done in a single step.  The following seems like it should work, but doesn't.  Apparently it does work like this in other languages' implementations of DataFrame.  Any thoughts?</p>
  
  <pre><code>using DataFrames
  df = DataFrame()
  df[:A]=[ 1, 3, 4, 7, 9]
  df[:B]=[ "a", "c", "c", "D", "c"]
  df[(df[:A].&lt;5)&amp;&amp;(df[:B].=="c"),:] 
  
  type: non-boolean (DataArray{Bool,1}) used in boolean context
  while loading In[18], in expression starting on line 5
  </code></pre>