Clarify ISO8859-1 limitation of __ReadableSequence

isReadableSequence and getReadableSequence both operate under the
assumption that the given buffer may be parsed as an array of
of 8bit single-byte coded characters (i.e. ISO/IEC 8859-1, _non_
Unicode). [Note that Unicode will need to be supported in future
versions]
Stub out Unicode version of getReadableSequence
Improve structure for reading of ID3v2 tag frames

Also stub out readId3v2TagFrameContentTxxx for reading user defined
text-information frames
Implement read user defined t.i. ID3v2 tag frames
Update docs
Beauty pass
Bump version patch number -> v0.1.5