httpcache: allow to import submodules within packages as HTTPCACHE_DBM_MODULE setting.
httpcache: added tests for custom dbm module.

And removed the break in the line to improve readability as there
is no strict 80-chars line width convention.
tests: dummydbm.error should be KeyError.
Merge pull request #234 from darkrho/httpcache-dbm-module-fullpath

allow to use full path in HTTPCACHE_DBM_MODULE setting