Added the all and any flags for keys assertion, with all being the default behavior

Extended keys documentation and set bools instead of changing flags in the keys assertion

Miswrote in keys documentation, corrected mistake

Edited documentation for keys to better match how it interacts with contains and have
Merge pull request #313 from cjqed/254-expect-any-all

Added the 'any' and 'all' flags for 'keys' assertion, with 'all' being the default behavior