preamble of index file is now lowercase

Decided to go for lowercase preambles, i.e. "index" or "track" instead of "INDEX" or "TRACK".
added read_value with stream value arg

Added another version of read_value to Generic which returns the field name as an std::string but buffers the field value into a caller-provided std::ostream. Should speed certain things up a bit.
added Track file class

Similar to db::file::Index, the new db::file::Track class reads and writes the data structure representing a track. This includes the track's id, metadata and a list of its different media formats.

Replaced the part of db-add.cpp that writes the track file with the above class for simplicity.