Adding Transaction Section

This pull request adds the transaction section to the devguide, along with SVG images to the /img/dev directory.

I recommend that comments on the pull request focus or errors, omissions, confusions, and other issues which should be fixed first.  Grammar and style checks can be done later.

Note: I fixed a mistake in two of the images, but GitHub is currently serving old versions of the images from its cache, so if you see "Secondary Script", know that it's been changed to "RedeemScript".