(maint) Add detailed docs on GEM_HOME and JRuby REPL's

Without this patch the developer documentation does not inform the
reader how to install Ruby gem dependencies when running in development
mode, and how this process is subtly different from installing Gems for
use with JRuby spec tests and for use with a packaged `puppetserver`
install.  This is a problem because developers may need to install
support libraries such as `pry` in an effort to get a REPL much the same
way the lein repl is used and it is not clear how to accomplish this
given the three scenarios of from packages (production), development,
and testing.

This patch addresses the problem by providing detailed information about
how to install gem dependencies when running puppetserver from source
and how the process differs from installing gem dependencies for testing
and for production packages.

The change also addresses a known issue when running the pry repl from
within the lein repl where by STDIN is not shared between the two and
JRuby REPLs cannot read user input.

This patch also changes all occurrences of `$ puppetserver gem install`
to `$ sudo puppetserver gem install` because the former does not work as
expected as described in SERVER-263.
On Wed, Dec 17, 2014 at 1:49 PM, Preben Ingvaldsen <notifications@github.com>
wrote:
>
> In documentation/dev_debugging.markdown:
>
> >
> > -    $ puppetserver gem install pry
> > +    $ sudo puppetserver gem -- install pry --no-ri --no-rdoc
>
> Huh, I thought that was fixed with commit 794035a. @jeffmccune What's the
output when you run it without the --?


You're right, I was confusing the behavior of `lein` and `puppetserver`
with regard to the gem command.

I've fixed it up.

The output is:

```
[vagrant@localhost ~]$ sudo puppetserver gem -- install pry --no-ri
--no-rdoc
RubyGems is a sophisticated package manager for Ruby.  This is a
basic help message containing pointers to more information.

  Usage:
    gem -h/--help
    gem -v/--version
    gem command [arguments...] [options...]

  Examples:
    gem install rake
    gem list --local
    gem build package.gemspec
    gem help install

  Further help:
    gem help commands            list all 'gem' commands
    gem help examples            show some examples of usage
    gem help platforms           show information about platforms
    gem help <COMMAND>           show help on COMMAND
                                   (e.g. 'gem help install')
    gem server                   present a web page at
                                 http://localhost:8808/
                                 with info about installed gems
  Further information:
    http://guides.rubygems.org
```