Renamed files & fixed includes: SimpleTag->Tag; DataTag->MultiTag
Renamed tags in all header files
Renamed tags in all source files and renamed I*Tag files
Renamed all instances of simple tag strings to tag, see below

Renamed "simple tag", "simple_tag", "SimpleTag", "SIMPLE_TAG"
Renamed simple tag in tests
Replaced last instances and variations of "data tag" & "simple tag"

including now "dtag" & "stag"
Renamed another single instance of "simpletag"
Merge pull request #339 from balint42/rename_tags

Rename tags