Allow subtypes for btceapi.keyhandler.KeyHandler

This would also allow subtyped btceapi.keyhandler.KeyHandler instances. An example of a subtyped KeyHandler class might be one that defines a close() function so the KeyHandler can be used with the 'with' syntax.

```
#!/usr/bin/env python

from contextlib import closing
import btceapi
import mybtceconfig

class ClosableKeyHandler(btceapi.KeyHandler):
	def __init__(self, filename):
		super(ClosableKeyHandler, self).__init__(filename, True)

	def close(self):
		self.save(self.filename)

with closing(ClosableKeyHandler(mybtceconfig.keyFile())) as keyHandler:
	tapi = btceapi.TradeAPI(mybtceconfig.key(), keyHandler)
	info = tapi.getInfo()
	print 'Current balance (USD): %s' % info.balance_usd
```