added string join and list concatenation SpEL methods

[vascofg]

This adds some methods I found necessary to avoid some crappy hacks on the pipelines:

• join('separator', list) takes a list, converts all items to strings and joins them by the provided separator
• concat(list1, list2) takes two lists and concatenates them together
• flatten(list) takes a list of lists and flattens it

[runabol]

Very cool. Thanks @vascofg!