implemented request tag logging to dumpfile
added documentation on request tag (and transaction) logging to dumpfile
default request tag to 'undefined' and log empty string to dumpfile
'request tag' -> 'tag'
default request tag to "" to avoid logic in ts_http:dump
fixed documentation on dumpfile format (request tag -> tag)
Merge pull request #62 from tisba/request-tag-logging

Logging of request tags to dumpfile