Updated String API and Unit Tests

Added additional unit tests for String objects, including equivalence
testing for Ascii, Latin1 and Utf8, all of which are currently
passing. It's arguable that all of the fromEncoding tests are already
equivalence classes for encoding - utf16, and so additional utf16
equivalence tests are likely not needed. A little bit more work on the
String API will be followed with audio device connectivity for the
Windows platform