Fix for boolean handling in whoosh backend (issue #371)

I reporded a bug yesterday that there might be a problem with how booleans are stored and queried in the whoosh backend. 

I noticed that whoosh stores booleans as the Python values True and False, haystack in contrast stored the strings 'true' and 'false', both evaluate as True if queried. 

In addition querying my index required to use the strings 't' and 'f' for boolean fields. With this patch it is possible to query with True or False.