BaGLAMa: add Houghton Library

[Federico Leva]

For evaluation of their wikipedian in residence work (which I may want to replicate in my current similar position), it would be very useful to have some stats about the files containing https://commons.wikimedia.org/wiki/Institution:Houghton_Library , or being in https://commons.wikimedia.org/wiki/Category:Media_in_the_Houghton_Library,_Harvard_University and subcategory, or uploaded by User:Rob at Houghton... whichever is easier for you.

I tried TreeViews but it didn't manage to complete the request.

[Magnus Manske]

Added.