Added NoDeduplicate field for hash & skiplist index options

[ewoutp]

Fixes #48

The added field is NoDeduplicate and not Deduplicate to avoid breaking changes.
Since the default value for deduplicate is true, and the go field will default to false if left unspecified, you would get unexpected deduplicate=false behavior when just updating the go driver without updating your code.

See also arangodb/arangodb#2644