Make the action data available to the filter script

This commit adds some context variables coming from the action meta data of the bulk item to filter scripts: action, _index, _type, _id, _version, _parent etc.

Closes #84
Related to #83
Merge branch 'pr/85' into es-1.4
Attempt (yes, attempt) to fix unit tests
(cherry picked from commit 9b354e4)