fix(server): un-nest arg checking

simplify the nesting in AddPeer
chore(README): add the travis badge lower

the travis badge was located above the title which looked a bit weird,
move it down.
chore(README): add an introduction section to the Overview

Add an introduction to the overview and put a link at the bottom.
feat(README): reorganize assuming someone knows raft

People who find this repo probably already know a bit about raft. So,
lets tell them about the current state of the project, the features that
are implemented and the example projects first.
chore(README): minor grammar fixes

Reread the README after my PR and caught a few things:

- a Go not an Go
- fixup an incomplete phrase
- simplify a sentence
chore(*.go): fix imports for protobuf

Fix all of the imports to move from benbjohnson to goraft. Thanks to Ben
for making this move to a new github org.
fix(misc): update to new URL

move README and Makefile to use the new URL.