Cache first call of DisplaySignatures. It only needs to be checked once.

Improves performance slightly on thread views, instead of calling up
the parents every single post, it just checks once to see if we can
display signatures and uses the cached value from then on in the same
request.
Merge pull request #97 from halkyon/displaysig_cache

Cache first call of DisplaySignatures. It only needs to be checked once. Passes 3.1 core tests.