Oauth library doesn't like it when you set request body to None

When I run the test_v2.py file I get the traceback pasted below. The oauth2 library doesn't handle a request body of None very well. The following pull request fixes this issue by setting the request body to an empty string rather than None.

	Creating v2 Etsy API, base url=http://openapi.etsy.com/v2.
	Not using cached method table.
	Traceback (most recent call last):
	  File "./test_v2.py", line 51, in <module>
		etsy_api = Etsy(config.api_key, etsy_oauth_client=oauth_client, etsy_env=etsy_env, log=my_log)
	  File "/usr/local/lib/python2.7/dist-packages/etsy/_v2.py", line 21, in __init__
		super(EtsyV2, self).__init__(api_key, key_file, method_cache, log)
	  File "/usr/local/lib/python2.7/dist-packages/etsy/_core.py", line 262, in __init__
		self._get_methods(method_cache)
	  File "/usr/local/lib/python2.7/dist-packages/etsy/_core.py", line 272, in _get_methods
		ms = self.method_cache.get()
	  File "/usr/local/lib/python2.7/dist-packages/etsy/_core.py", line 179, in get
		ms = self.api.get_method_table()
	  File "/usr/local/lib/python2.7/dist-packages/etsy/_core.py", line 286, in get_method_table
		return self._get('GET', '/')
	  File "/usr/local/lib/python2.7/dist-packages/etsy/_core.py", line 328, in _get
		data = self._get_url(url, http_method, content_type, body)
	  File "/usr/local/lib/python2.7/dist-packages/etsy/_v2.py", line 25, in _get_url
		return self.etsy_oauth_client.do_oauth_request(url, http_method, content_type, body)
	  File "/usr/local/lib/python2.7/dist-packages/etsy/oauth.py", line 43, in do_oauth_request
		resp, content = self.request(url, http_method, body=body)
	  File "/usr/local/lib/python2.7/dist-packages/oauth2/__init__.py", line 662, in request
		req.sign_request(self.method, self.consumer, self.token)
	  File "/usr/local/lib/python2.7/dist-packages/oauth2/__init__.py", line 493, in sign_request
		self['oauth_body_hash'] = base64.b64encode(sha(self.body).digest())
	TypeError: must be string or buffer, not None