Threadsafe Recurly.api_key

Very straightforward solution, which brings a undesired behaviour - code, that relies on Recurly.api_key being set by some other thread,  will fail.
So, second commit to fix this issue, but for a main thread only - if you set Recurly.pai)key in the main thread, it will be avail to all threads run afterwards. 
But if any of these threads sets its own key - it will use its own key.