title: drop lmdb database using python
body: <p>I would like to drop an lmdb database.  The following hits 'typerror:invalid type' on the txn.drop(db_name) line.</p>
  
  <pre><code>import lmdb 
  def kill_db(db_name):
      print('ABOUT TO DELETE DB '+str(db_name)+'!!!!')
      raw_input('press enter to continue or ctrl-C to not')
      env = lmdb.open(db_name, readonly=False)
      txn = lmdb.Transaction(env)
      txn.drop(db_name)
  </code></pre>
  
  <p>any clues as to why this is happening? 
  I am new to lmdb (as you may have guessed) so be gentle.</p>