Add objects/set.{c,h} files and CMakeLists.txt entry
Add initial interface definition
Add stubs for ws_set type implementation
Add implementation for ws_set_init()
Add implementation for ws_set_new()
Add implementation for ws_set_insert()
Add implementation for ws_set_union()
Add implementation for ws_set_insertsection()
Add implementation for ws_set_xor()
Add implementation for ws_set_is_subset()
Add implementation for ws_set_equal()
Add implementation for ws_set_cardinality()
Add implementation for ws_set_select()
Outcommented functions which libreset does not ship ATM
Add implementation for ws_set_remove()
Add implementation for ws_set_get()
Merge pull request #86 from matthiasbeyer/add-set_type

[WIP] Add set type