[Bundler] Use Bundler to manage gems (for development)

This was added in the past, but it apparently caused problems so was
disabled (instead of fixing the actual issues).

Using the official code from http://gembundler.com/v1.3/rails23.html.

This change makes the development workflow much more sane - just update
the Gemfile and `bundle install` to install and develop against the
specific gems and versions. This is particularly useful when
adding, testing, updating, or removing Gems. We can also be sure that
(eg. with `bundle exec`) we're ONLY using the specified set of Gems and
corresponding versions.

The product will still use Gems installed from RPMs (NOT `bundle
install`), so we need to keep the Gemfile updated accordingly.
[Bundler] Use 'https://rubygems.org' instead of :rubygems

Avoids deprecation warning (with Ruby 1.9) and is more secure.
[Bundler] Remove unused gems: thin and formtastic
[Bundler] Specify exact Gem versions

This ensures that we use the same Gems both in development, where one
may use `bundle install`, and in the product, where they are installed
via RPMs.

Well, more or less the same since the RPM Gems, even if of the same
version, may contain additional patches, but this is rare and even so
usually does not affect the behavior.

We'll need to manually update the version strings here to match what we
package and release as RPMs, which should be fine since they don't
change much.
[Bundler] Add gems needed for running in production mode
[Bundler] Disable use of Bundler by default

There are many concerns that the re-introduction of Bundler, especially
at this stage of the release, will cause more problems than benefits.
But it is still very useful to have the ability to use Bundler,
particularly during development. So let's take the middle ground and
push the code in, but disable it by default.

See the discussion at
https://github.com/crowbar/barclamp-crowbar/pull/557 for details.

* Use the `app_config` gem [1] for configuring the use of Bundler (off
  disabled by default). Will be used for other config options as well.
  It's similar to the `rails_config` gem [2] that's already used in
  Crowbar 2.x, but works for Rails 2 (rails_config doesn't).

* Gemfile and Gemfile.lock are ignored by default, and only parsed when
  `use_bundler` is set to `true` (which is very useful during
  development).

[1] https://github.com/Oshuma/app_config/tree/v1.0.2
[2] https://github.com/railsjedi/rails_config
Merge pull request #557 from jamestyj/release/pebbles/master-bundler

Use Bundler (again, but disabled by default)