'Bookingit' fixes for diffing, running commands, and readme.

Hi Dave, when I tried to generate the angular-rails HTML, I encountered a few issues on my machine, and have have some proposed fixes for your consideration.  

The problems I encountered were:
1. `git diff` caused opendiff to launch, since I have the diff.external config set to use opendiff
2. bookingit was not using the receta gemset when invoking commands (it was using the inherited angular-rails-book gemset)
3. database commands failed since receta/config/database.yml specifies 'receta' as the PostgreSQL user
4. Firefox would hang when running tests
5. The .ruby-version and .ruby-gemset were missing for tags within the `receta` depot, which caused 'rake spec' and other commands to fail

I have attempted to rectify these issues, and the changes span three repos (angular-rails-book, bookingit, and receta).

In the `bookingit` repository:

* [1] Direct `git diff` to ignore the diff.external config value

* [2] Before invoking a command in `bookingit`, use sub-repository's .ruby-version & .ruby-gemset

In the `angular-rails-book` repository:

* [3] add line to README which directs the user to run `createuser` to create new PostgreSQL user

In the `receta` repository:
* [4] Update selenium-webdriver gem version from 2.40.0 to 2.44.0
* [5] Replace *twelve* of the tags with branches, and, to each of these new branches, add .ruby-version & .ruby-gemset and update selenium-webdriver version.

Unfortunately, fix [5] requires a seperate pull request for each new branch.  However, using seperate branches instead of tags provides a couple benefits since it is backwards compatibale with the tag definitions in the markdown files and would be useful for adopting future toolchains (e.g. upgrading to Ruby 2.1.5, the latest Rails, or other gems). 

Please review at your convenience.

-Doug