Add stringlist_pop func and tests.

- I've added a stringlist_pop function as this is needed for the
language tag implementation.
- I've also added the necessary tests
for the new function.
- Also added tests for other string list functions that don't have
tests yet.
- Updated string c++ API.
Merge pull request #396 from RickEyre/stringlist-pop

Add stringlist_pop func and tests.