Give example for adding calculator

Believe or not but I struggled a bit with adding my own calculator to spree. As some other folks:
#1: http://grokbase.com/t/gg/spree-user/141amnc5ns/creating-spree-extension-uninitialized-constant
#2: http://stackoverflow.com/questions/5119960/how-to-properly-load-a-handmade-extension-in-spree
#3: https://www.ruby-forum.com/topic/4403156

It's all about naming convention so why not give example just in case someone forgot it (as I did!)
Since Jan 2013 QST is no longer compound.

http://www.revenuquebec.ca/en/entreprise/taxes/tvq_tps/calcul-taxes.aspx

Fixes #237
Fix checkout payment example

Fixes https://github.com/spree/spree/issues/4858
Use Rails 4.0.5

Fixes #238
Bump Rails to 4.1.2
Remove Activators guide + update promotions guide
Add 2.2.0 release notes
Add 2.2.0 release notes to sidebar
Add 2.1 -> 2.2 upgrade guide

Finally.
Add 2.1 -> 2.2 upgrade guide to sidebar
Add 2.3 release notes + upgrade guide
Update Release Policy for 2.3 release
fixes some typos and formatting in 2.3 release notes

Fixes #242
Adding possessive apostrophe
Changed order of core docs nav
Merge pull request #244 from spree-girls/core_nav_reordering

Changed order of core docs nav
Update markdown for ruby coded examples

construction <% ruby do %> ... <% end %> was changed to ```ruby ... ```

Fixes #240
Extension doc clarification

Fixes #239

```
3:29 PM <BAMbanda> I have a question about installing extensions
3:30 PM <BAMbanda> according to the guide, we are suppost to run a rails generated migration. Do we do that in our application directory, or in our "simple_sales" extension director
3:30 PM <BAMbanda> http://guides.spreecommerce.com/developer/extensions_tutorial.html
3:31 PM ⇐ chris_hmc quit (~chris_hmc@75-171-88-228.phnx.qwest.net) Ping timeout: 248 seconds
3:34 PM <dpritchett> BAMbanda: is it your extension or someone else's?
3:34 PM ⇐ beneggett quit (~beneggett@c-71-199-62-122.hsd1.ut.comcast.net) Quit: My MacBook Pro has gone to sleep. ZZZzzz…
3:34 PM <BAMbanda> dpritchett, it's mine, I created manually, following the guide
3:35 PM <dpritchett> usually when i use a spree extension i 1) add it to my spree store's gemfile, 2) bundle install, 3) run the extension installer if necessary, 4) run the extension railties to generate migrations if necessary 5) run rake db: migrate if necessary
3:35 PM <dpritchett> all this without ever messing with the extension
3:35 PM <dpritchett> since it's your own extension you should know if you built an installer or any railties or whatever
3:35 PM <dpritchett> but yeah you run all of that from within your spree app's context and not that of the extension
3:36 PM <dpritchett> assuming your extension is self-contained in another folder outside of your app repo
3:36 PM <BAMbanda> ok cool, I generated it though the spree comandline gem, outside my apps repo
3:36 PM <dpritchett> rerun it inside your app's repo and it should put the migration in db/migrate
3:36 PM <dpritchett> or you could just copy it over
3:36 PM <dpritchett> (i think)
3:37 PM <BAMbanda> yeah, that makes sense, ok thanks
3:37 PM <dpritchett> i'm assuming you had a file like this in your extension: https://github.com/spree/spree_gateway/blob/master/lib/generators/spree_gateway/install/install_generator.rb
3:38 PM <BAMbanda> yup I did
3:38 PM <dpritchett> cool
3:38 PM → chris_hmc joined (~chris_hmc@75-171-88-228.phnx.qwest.net)
3:38 PM <BAMbanda> so then in my app, I ran the generator (rails g spree_simple_sales:install)
3:40 PM <•GeekOnCoffee> BAMbanda: pull requests welcome to make things more obvious: https://github.com/spree/spree-guides/blob/master/content/developer/tutorials/extensions_tutorial.md and https://github.com/spree/spree/blob/master/cmd/lib/spree_cmd/templates/extension/README.md
3:43 PM <BAMbanda> @GeekOnCoffee, thanks
```
fix Ability namespace on security tutorial
Merge pull request #245 from alexandremcosta/ability_namespace

fix Ability namespace on security tutorial