Work around transaction search timeouts in the client

[willglynn]

As promised, here's a client-side fix to the transaction timeout search issue in #46. It works by catching the 422 Unprocessable entity responses and retrying with two smaller requests -- recursively.

I believe #55 is the right solution, but this would work too.

[braintreeps]

Hey @willglynn,

Thanks for another pull request. As we said in #55, updates to speed up our search API will be released soon. We're aware that this is a continuing problem and are taking action to fix this.

Katelyn and Lionel
Braintree Developers