The "Why" For Standard Transactions, Plus Std Tx Tweaks

Based on a discussion tonight on IRC with comments from @gmaxwell,
bitcoin428, and kadoban.

* Add a new introduction to the Standard Transaction section which
  explains the purpose behind standard transactions so readers
  understand that these aren't limitations for limitation's sake. More
  specifically, note that bug-prevention isn't the only reason for
  standard transactions---thanks to @gmaxwell for explaining the forward
  compatablity reason to me.

* Make it clear that isStandard() only applies to loose transactions and
  that it doesn't apply to txes in blocks.  Thanks to bitcoin428 for
  pointing out this source of confusion.

* Make it cleare that a non-standard redeemScript is a script that would
  not pass isStandard().  Thanks to kadoban for pointing out the absense
  of that information.