(MODULES-444) Add specs for new behaviour

`concat` can now take multiple arguments
(MODULES-444) Acceptance test for primitives

`concat` should be able to concat arrays and primitives
(MODULES-444) Acceptance for multiple arrays

Acceptance test to take multiple arrays for concatenation
(MODULES-444) Change test to > 2 arguments

Also add extra test for just 1 argument
(MODULES-444) - Check for accepting > 2 args
(MODULES-444) Update docs with new functionality
(MODULES-444) Change argument restriction to < 2
(MODULES-444) - Real meat of the change

This is the core change, we now go through the array and add it to the first element, instead of just two arguments.
Merge pull request #374 from petems/MODULES-444-add_concat_multiple

MODULES-444-Add concat multiple