db-meta: removed blob listing

Since the db-blob.cpp tool provides a way to display and modify a track's blobs,
this functionality can be removed from db-meta.cpp.
fixed bug in db/file/{Index,Track}.hpp

If an index or track file consisted of only the header and had no actual tracks
or blobs listed, the read() functions would complain about an end of stream in
the middle of the data. An additional check whether skipping newlines produces
an end of stream resolves the issue.
db-add: added --remove option

The tool in db-add.cpp now provides a --remove option that allows tracks to be
removed from the index either by permanent id or hash name.
added raw Unix time output to common/Date.hpp

Tools now write Unix timestamps to disk instead of formatted dates. This should
simplify handling a bit.