Support leads/describe2.json API endpoint

[apellis]

Currently, node-marketo supports the leads/describe.json endpoint. Marketo has introduced the leads/describe2.json endpoint. According to their developer relations team, developers are encouraged to use describe2 instead of describe, at least because:

1. describe has "REST API names" and "SOAP API names". describe2 just has a single "API name". The SOAP API isn't encouraged for use anymore, and Marketo is just calling the REST API name the API name now.
2. describe has a notion of field id that isn't used anywhere else in the Marketo API. describe2 removes this (now useless) notion.
3. describe2 indicates which fields are searchable (i.e. can be used as query filter types in the "get leads by filter type" API endpoint).

node-marketo should support describe2 as well as describe. It's an easy change, so I'll submit a PR myself.