Fix wording for Step 1

Step 1 wording is too verbose considering we have already instructed the
student to run acommand in the terminal in Step 0 without all this
explanation.
Remove wrong instruction from Step 2

This guide is a continuation of the Rails Girls App Tutorial where the
student has already added a root route.
Simplify Step 3

Make Step 3 simpler and straightforward with clear instructions to
follow and a note for the coach to explian what the default user model
is
Simplify instructions for Step 4

Stuends can use the sign up form to create an account and don't need to
use the console. Also, the console example was wrong, since the default
pasword length must be at least 8 chars long and the example showed a
7-chars password.
Simplify Step 5

Clean erb examples, fix highlight to erb and add coach notes.
Credit for the updates
Clean layout in Step 2 with proper styles

Use the proper styles from bootstrap and add some ifs to hide the
paragraphs when there are no messages.
Merge pull request #57 from ernesto-jimenez/master

Devise guide updated