Move namedPosition up to Role from OrganizationRole

Per discussion on public-vocabs about adding a more generic roleName for the
Role type, vs. namedPosition living only on a subtype (OrganizationRole) of
Role:

"""
Promoting "namedPosition" to Role and renaming the property "roleName" or
the like sounds good to me.

- Vicki
"""

http://lists.w3.org/Archives/Public/public-vocabs/2014Sep/0331.html

Signed-off-by: Dan Scott <dan@coffeecode.net>
Complete the shift from namedPosition to roleName

Add roleName, supersed namedPosition, update examples.
Per http://lists.w3.org/Archives/Public/public-vocabs/2014Sep/0331.html

Signed-off-by: Dan Scott <dan@coffeecode.net>
Merge pull request #23 from dbs/sdo-rolename

sdo-rolename: have a roleName on Role instead of lots of similar specialized properties.

Addresses https://github.com/rvguha/schemaorg/issues/151