refactor the naming within plugin new generator

Having a method called `full?`, which checks on :full and :mountable
is very confusing. I renamed `full?` to `engine?` and created a `full?`
method that only checks the `:full` option
Updated link to RubyGems site
Update version numbers of Ruby and Rails
Modified "Allowing the update of fields" section

* Rails generate model Post ______ creates the
  attr_accessible line for you for any fields
  you specify. Changed the section to describe
  what this line in the model does.
Changed the Using Partials section to read a little better

* No need to give a simple example, as we have a
  partial that we create in the blog app
Added new line to make it more readable

* This shows the reader there is a method: and 
  data: parameters
Moved position of has_many
Updated with where functional/helper tests go
We should use post_path, not post_url
Reword Post model to note there are lines not shown
Revert "Updated with where functional/helper tests go"

This reverts commit c94cfdad6388dcdfaf4c10b9e8cee46baf2a7901.

* This is the new 4.0 (master) directory structure
Update guides/source/active_record_validations_callbacks.md

In the expiration_date_cannot_be_in_the_past validation method, use `expiration_date.present?` instead of the double negative `!expiration_date.blank?`. Also join the comparisons with `&&` instead of `and`, which could cause unintended consequences.
Merge pull request #119 from c0/master

Clarification to ActiveRecord Validations Doc in 7.2 Custom Methods
Set fixed-width style where appropriate on Getting Started guide.
correct css selector for the display of validation error messages: changed #error_explanation to #errorExplanation
Fix validation based on object not _id.

From https://github.com/rails/rails/issues/6161\#issuecomment-10750118
Adding a note about :inverse_of for validations.

From https://github.com/lifo/docrails/commit/cfd324b4b68469ba3188e4b7ba8586e59b239693\#commitcomment-2213592
Clarifies the cookie store docs a bit and uses correct Markdown syntax for preformatted text
Revert "correct css selector for the display of validation error messages: changed #error_explanation to #errorExplanation"

At least in master the selector is snake case.

This reverts commit ba2fed41613e02bb6cfc46c92ace00abb99ce313.
Use assert_nil instead of assert_equal