Move P2SH/Multisig back under Transaction

These changes are submitted to the "work-in-progress" devcontracts branch to address suggestions in https://github.com/saivann/bitcoin.org/pull/42#discussion_r11277653 on pull req #42

@harding I'm not sure this addresses your comment correctly, please let me know if you think this pull req needs to be scrapped or merged. If these changes are generally OK, we can merge them and comment them on pull request #42.