add Faker\Provider\Base::randomElements
optimize randomElements performance
make randomElements ~12% faster
Lower randomElements defaults and add readme a note

Also tidied up the LengthException message, and made randomElements
just a tiny bit faster by moving the array lookup to after a unique
random value is chosen.
Rewrite randomElement to use randomElements, and add test
Merge pull request #266 from terite/randomElements

add Faker\Provider\Base::randomElements