Add the ability to associate plaques with multiple organisations

[frankieroberto]

Copied from http://code.google.com/p/openplaques/issues/detail?id=23&q=organisation:

"Some plaques have more than one organisation listed (eg
http://news.bbc.co.uk/1/hi/entertainment/7932019.stm).

Currently, this relationship isn't supported."

[frankieroberto]

Added by Jez's awesome commit: 4220390 :-)