enable multiple value index for collection

pymongo supports ensure_index(key_or_list) where list can be [('key1',
direction,), ('key2', direction),]. This patch will allow scrapy-mongodb
to properly handle upserts when MONGODB_UNIQUE_KEY is a list of tuples
Merge branch 'sherzberg-multiple_index_keys' into hotfix/0.7.2
Bumped version number to 0.7.2
Updated release notes
Merge branch 'hotfix/0.7.2'