Use Terms "Pubkey Script" And "Signature Script"

This modifies two commits provided by @petertodd in #563 to use the terms "pubkey script" and "signature script" instead of other terms.

* Rename "scriptPubKey" and "output script" to "pubkey script" (suggested by @luke-jr). We leave a token "scriptPubKey" at the point where we define pubkey script so that searchers can find it.

* Rename "scriptSig" to "signature script" (suggested by @luke-jr). We also leave a token "scriptSig" at this definition point.

* Rename "redeemScript" to "redeem script"

* Defined ECDSA on secp256k1 curve as the crypto used and added references to secp256k1 private/public keys and signatures.

* Removed "The Parts Of A Transaction" illustration by commenting it out in the HTML. Shoehorning the pubkey/signature script terms into it was becoming difficult, and I'm not very fond of that illustration anyway.  I'll see if I can think of a nicer replacement illustration.

* Add a short paraphrased version @petertodd's description of scripts as generalized crypto. The exact sentence is: "Pubkey scripts and signature scripts combine secp256k1 pubkeys and signatures with conditional logic, creating a programable authorization mechanism."

* Updated all the illustrations which referred to either pubkey scripts or signature scripts.