Move JSON event generators into separate modules

This moves the JSON event generators out of the Stasis-HTTP modules and
into standalone JSON-related counterparts so that Stasis-HTTP and
res_stasis can depend on them without creating dependency cycles. This
also provides a future location for Swagger Model validator functions
once the generators for that code are written.

Review: https://reviewboard.asterisk.org/r/2534/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@388668 f38db490-d61c-443f-a65b-d21fe96a405b