Client: json-strict support

Added support for Cheshire's decode-strict, now triggered with
(client/get ... {:as :json-strict}). Note that :auto and
:json-string-keys both default to non-strict parsing (unchanged).

Cheshire 5.3.1 is required (5.3.0's decode-strict is buggy).

Fixes #173.
Now mentioning :json-strict in readme.
Tests: Using vector? instead of = on (type foo)
Merge branch 'pr/175'
Add {:as :json-strict-string-keys}