Move HostWhitelist, NewHostWhitelist, IsWhitelisted, IsEmpty, and Count
from internal/imgcache into the new internal/whitelist package.
The whitelist package is completely self-contained, depending only on
net/url and strings from the standard library.
Updated import in internal/imgcache/service.go to use the new package.
Tests moved and adapted to external test style (package whitelist_test).
Part of #39
