Made the error message when we encounter an unknown script pubkey more clear.
Revised error message per feedback on PR.
Merge pull request #492 from curtislacy/Pubkey-Error

Clarified error message when we encounter an unknown script pubkey
Merge pull request #493 from chjj/fix_paypro_example

Fix Payment Protocol Example
paypro: example - allow more modularity concerning options.
paypro: example - document modular usage of example.
paypro: example - fix customer createTX for browser.
paypro: example - remove old createTX.