removed some trailing whitespace
Rephrased what trac is good for.

I found the existing list hard to read, hope it's better now.
Added a section on how to upload/generate an ssh key with the dev scripts.
Added a small disclaimer to the walk through:

* you need an account on trac
* what to do if sage is not in your path
* what to do if sage -dev has disappeared
Some additional info for sage -dev create-ticket

* explained what to do with the fields and where to find a description for them
* the branch created by create-ticket is local
* rephrased the sentence about aborting creation
more details on setting a ticket to needs_review
removed a duplicate "existing" in a sentence