Pedantic const correction.

The string table in EncodingName was missing a const. It could never, ever
have been modified, but it is nonetheless corrected now.
Added empty modules for UTF-8 and its unit tests.

- Added Utf8Encoding module.
- Added TestUtf8Encoding.cpp to test UTF-8-specifics.
- Added TestEncodings.cpp for encoding-agnostic tests.