Give *Number series properties a consistent range

Per http://lists.w3.org/Archives/Public/public-vocabs/2014Aug/0140.html,
make issueNumber and volumeNumber accept a range of Integer as well as
Text, while relaxing episodeNumber and seasonNumber such that they will
accept Text as well as Integer.

Signed-off-by: Dan Scott <dan@coffeecode.net>
Make *Number subproperties of "position"

Per http://lists.w3.org/Archives/Public/public-vocabs/2014Aug/0140.html,
make episodeNumber, seasonNumber, issueNumber, and volumeNumber
subproperties of position, and make CreativeWork the domain for
position. Adjust the definition of position accordingly.

Signed-off-by: Dan Scott <dan@coffeecode.net>