Add support for low-level batched requests

[benmvp]

The API supports batched requests so the SDK should support it as well to make things convenient.

We'll flesh out details of the JS API here. Should be done after #6

[ryanwholey]

It is worth noting that the Eventbrite v3 default batch process only applies to GET requests. Every Eventbrite api v3 endpoint supports batch GET. Batch update it is still experimental, however many data sets that need batch create update delete support have their own batch implementations separate from the standard v3 batch process.

[benmvp]

Ahhh, good to know. So we can at the very least support batch GET

[kwelch]

Batched requested docs have moved here => https://www.eventbrite.com/platform/api#/introduction/batched-requests