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.
Standard Tx Modifications

* Remove contentious sentence about mining non-standard txes possibly
  creating orphaned blocks and loss of block reward.  Suggested by
  @luke-jr (thanks!)

* Remove unnecessary value judgement ("unfortunately") against
  non-standard redeemScripts. Mention that they can be used deliberately
  by someone who wants to easily receive payment to a non-standard
  output script. Suggested by @luke-jr (thanks!)
Merge remote-tracking branch 'origin/standardtxes'