Initial support for native protocol v2

No new features added.
Add v2 authentication request/response messages
Support for query parameters (e.g. paging, serial consistency)
Add support for BATCH request
Some irrelevant changes
Add support for batch request

Right now it only supports prepared queries.
Bump to 0.2.0-pre1.
Return name and query when erlcql_client:prepare/3 fails

Fixes #26