Fixed comment

-TagFactory's constructor's summary incorrectly stated that it wasn't
used.
Removed redundant TagReader passing

-TagFactory.DevBuildTag now gets its reader out of the TagData instead
of requiring it to be passed separately.
Raindrop constructor fully initializes its TagData

-The TagData made by the Raindrop constructor wasn't fully initialized
and may have caused hard to debug errors if the missing values were
used.
Simplified TagStruct construction

-Most tags call Helpers.BuildTag to build a default tag now. This makes
construction simpler and makes it more likely that changes to the
TagData will be corrected everywhere.
Comment fixes

-EofTag.Build's comment no longer implies that the value-type TagData
can be null.
-EofTag.Build's comment is now more clear about the fact that it ignores
its TagData on purpose and why.
-Removed another TODO comment about something that was already finished.