Added searchhistogram.json to the API requests

Enabled requests with the call:

results = Topsy.search_histogram( query , "citation" , 86400 , 30 )

or 

results = Topsy.search_histogram( query , "target" , 86400 , 30 )

Request documentation found here: http://code.google.com/p/otterapi/wiki/Resources#/searchhistogram