Go through curriculum with a fine-tooth comb

I went through our old curriculum at railsbridge-boston/railsbridge-boston and this curriculum, step-by-step, to add missing information and fix errors in this curriculum.

Many of the commits are formatting, like adding backticks, removing quotes, fixing whitespace, or fixing grammar collisions. I've summarized the Big Changes, with commit hashes, below.

I welcome any feedback, positive or negative!

* cf524b3 Everyone's in a VM, so we can use `cd ~` for everyone, instead of having Windows/Mac/Unix options.
* 5058adc, d8a6766 Encourage good commit messages and variable names
* 68c8a65 Tell people to sign up for Heroku before running `heroku create`
* 064cfd8, ac3623b Remove inappropriate language like "spanking" or "hawt"
* 45dc862 Don't give any recommendations about Rails 3 since we know students will use Rails 4
* 13f4a19 Add an extra credit CSS-styling step at the very end of the curriculum
* ed11567 Add an optional checkpoint so people can double-check their Ruby skills before starting
* 4e0f8f2 Encourage using `link_to ..., topic_path(topic)` over `link_to ..., topic` so people can mentally connect that to routes
* dddf713 cd to `~/workspace` in the VM instead of `~` - I think this was a big problem at previous workshops, since it meant students' work was not available outside of the VM
* 4ac6cb7ab Explain the format of a migration file
Oh, I see now. It wasn't nested in the old curriculum, but I agree that they're related. I'll fix it without nesting.