Added default wide-character typedefs

Added `immutable_wstring`, `immutable_u16string`, and
`immutable_u32string` typedefs to match the default typedefs for
`std::basic_string`. Also, made `.mutable_string()` return the proper
string type.
Corrected spacing
Pull request from https://github.com/xymostech/cpp_immutable_string

Merged branch 'xymostech-wide-char-typedefs' into develop
Merge branch 'develop'