Fix how the uwsgi plugin fetch data: make sure it pull the entire json and not the first chunk

The uwsgi plugin reads  the stats socket only once before loading the json message. The plugin crashes when uwsgi does not finish writting the entire message trough the socket.

Here is the traceback I got (the plugin onlyu pulled 2/3 of the message):

	INFO       2013-09-05 17:54:57 678    MainProcess     MainThread newrelic_plugin_agent.plugins.base            poll                      L272   : Polling uWSGI
	Exception: ValueError('Expecting , delimiter: line 116 column 7 (char 1665)',)

	Traceback (most recent call last):
	  File "/usr/local/lib/python2.6/dist-packages/clihelper/__init__.py", line 744, in run
	    process.run()
	  File "/usr/local/lib/python2.6/dist-packages/clihelper/__init__.py", line 388, in run
	    signal.pause()
	  File "/usr/local/lib/python2.6/dist-packages/clihelper/__init__.py", line 577, in _wake
	    self.process()
	  File "/usr/local/lib/python2.6/dist-packages/newrelic_plugin_agent/agent.py", line 110, in process
	    self.start_plugin_polling()
	  File "/usr/local/lib/python2.6/dist-packages/newrelic_plugin_agent/agent.py", line 239, in start_plugin_polling
	    self.application_config.get(plugin))
	  File "/usr/local/lib/python2.6/dist-packages/newrelic_plugin_agent/agent.py", line 100, in poll_plugin
	    thread.run()
	  File "/usr/lib/python2.6/threading.py", line 484, in run
	    self.__target(*self.__args, **self.__kwargs)
	  File "/usr/local/lib/python2.6/dist-packages/newrelic_plugin_agent/agent.py", line 254, in thread_process
	    obj.poll()
	  File "/usr/local/lib/python2.6/dist-packages/newrelic_plugin_agent/plugins/base.py", line 282, in poll
	    data = self.fetch_data(connection)
	  File "/usr/local/lib/python2.6/dist-packages/newrelic_plugin_agent/plugins/uwsgi.py", line 95, in fetch_data
	    return json.loads(data)
	  File "/usr/lib/python2.6/json/__init__.py", line 307, in loads
	    return _default_decoder.decode(s)
	  File "/usr/lib/python2.6/json/decoder.py", line 319, in decode
	    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
	  File "/usr/lib/python2.6/json/decoder.py", line 336, in raw_decode
	    obj, end = self._scanner.iterscan(s, **kw).next()
	  File "/usr/lib/python2.6/json/scanner.py", line 55, in iterscan
	    rval, next_pos = action(m, context)
	  File "/usr/lib/python2.6/json/decoder.py", line 183, in JSONObject
	    value, end = iterscan(s, idx=end, context=context).next()
	  File "/usr/lib/python2.6/json/scanner.py", line 55, in iterscan
	    rval, next_pos = action(m, context)
	  File "/usr/lib/python2.6/json/decoder.py", line 217, in JSONArray
	    value, end = iterscan(s, idx=end, context=context).next()
	  File "/usr/lib/python2.6/json/scanner.py", line 55, in iterscan
	    rval, next_pos = action(m, context)
	  File "/usr/lib/python2.6/json/decoder.py", line 183, in JSONObject
	    value, end = iterscan(s, idx=end, context=context).next()
	  File "/usr/lib/python2.6/json/scanner.py", line 55, in iterscan
	    rval, next_pos = action(m, context)
	  File "/usr/lib/python2.6/json/decoder.py", line 227, in JSONArray
	    raise ValueError(errmsg("Expecting , delimiter", s, end))
	ValueError: Expecting , delimiter: line 116 column 7 (char 1665)

The following patch fixes it.