global secondary index support
Merge commit 'ae6317b7aa4a5bca8815b1bc8f8729b2c217249c' into gsi

Conflicts:
	src/erlcloud_ddb2.erl
documentation added and a missing type export added
index status is an atom
added test cases for global secondary index creating and describing
type spec for index_status
fix index_status field type
Merge pull request #89 from gleber/pr88

Pr88
Merge commit 'ae2a4a2e7779c2fb2d45fd8fa37f3aa5f5cc815c'