Add new config options: enabled + recording

[watson]

To align with the other agents and to provide greater flexibility, we're introducing two new config options: enabled and recording, while deprecating active.

Here's the expected behavior as described in elastic/apm#92 (comment):

• enabled - Should behave identically to active, which in turn should be deprecated (this is essentially a rename to get a more meaningful name)
• recording - Should default to true. Setting to false will completely disable the recording and sending of data (transactions, spans, errors, and metrics). It must be possible to toggle recording dynamically, either locally or via remote config. Setting recording to false will therefore not disable remote config polling, config file watching, etc.