Allow passing an array of ids to fetch_multi
Read from cache in batches

Prevent SystemStackError being raised when the number of keys is greater than the max stack size
Flatten keys only one level in fetch_multi
Fix deprecation warning in cache_runner.rb
Improve testing of #fetch_multi "reads in batches" behavior
Document passing an array to fetch_multi and update CHANGELOG
Add regression test for fetch_multi stack level error
Merge pull request #141 from Thibaut/fetch-in-batches

Fetch in batches
Fix query api for custom primary key name.

Fixes #142