wa_api.cc: Add explicit type casts

We need to cast C strings to std::string before passing them to the
WhatsApp protocol engine. Add missing explicit casts.

Signed-off-by: Lukas Fleischer <info@cryptocrack.de>
Use proper default resource

Use a proper default resource instead of misusing the user agent string
as a resource string. The default value is taken from the yowsup
sources. This hopefully fixes issue #37.

Signed-off-by: Lukas Fleischer <info@cryptocrack.de>
Merge pull request #44 from cryptocrack/fix-default-resource

Fix default resource