Removed spaces after periods, edited 1st section

See the entry I added to the style guide regarding spaces after periods: https://docs.google.com/document/d/1J_gHhbZXaliFEC9Ti-Rmky-9NSyKBeUzKXzHWvqnIvw/edit#
Editing up to "Proof of Work"

I did some line-rewrapping as well. I hope that doesn't screw things up by making the diff unnecessarily complicated....
Finished general edit by TMG

General comments to be discussed elsewhere.
Changed "satoshis" to "value"
Resolve merge conflict
Remove double quotes on "mutable"
Update developer-guide.md

As per discussion, break into two parts the paragraph containing "spenders propose a transaction fee"
Update developer-guide.md

s/return an item of value/send an item of value/
Merge pull request #8 from saivann/merge-tgeller

Merge tgeller's changes; reviewed by harding and saivann with additional feedback from tgeller.