Add string.c to CMakeLists.txt
Add ws_string interface

Add definitions for string.h
Add ws_string method stubs
Add implementation for ws_string_init()
Add implementation for ws_string_new()
Add implementation for ws_string_len()
Add implementation of ws_string_cat
Add implementation of ws_string_multicat
Add implementation of ws_string_dupl
Add implementation of ws_string_cmp()
Add implementation of ws_string_ncmp()
Add implementation of ws_string_substr()
Add implementation of ws_string_raw()
Merge pull request #52 from xVin/add-string_type

Add string type