headers: New methods on NcxRecord

This commit adds toByteArray() and addEntry(String) to NcxRecord so that
text index records can be built.
headers: New constructors on TagxTag

The TagxTag class holds bytes used to decode the index record. The
existing constructor is designed for extraction and we're closing in on
being able to build a functional index, this commit adds constructors to
TagxTag to allow building them with definite parameters.