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
`assert_template` fails with empty string when a template has been rendered

For instance, it prevents false positive in this case:

    file = nil
    get :index
    assert_template("#{file}")
`assert_template` fails with empty string.
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
cleanup, remove broken whitespace
cleanup, removed dispensable `require` statements from `ActiveModel`
Revert "Updated with where functional/helper tests go"

This reverts commit c94cfdad6388dcdfaf4c10b9e8cee46baf2a7901.

* This is the new 4.0 (master) directory structure
Merge pull request #8302 from roberto/assert_template_empty_string

assert_template("") will now fail no matter whether a template has been rendered or not.
assert_template: validating option keys
Add explicit opt-out for fragment cache digesting

This add support for sending an explicit opt-out of the "Russian-doll"
cache digest feature on a case-by-case basis. This is useful when cache-
expiration needs to be performed manually and it would be otherwise
difficult to know the exact name of a digested cache key.

More information: https://github.com/rails/cache_digests/pull/16
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