add a type to test inheriting from ini_setting

In order to test with rspec-puppet we need to set up the fixtures, and test via
rake spec, so we change that too
fix Gemfile for current bundler

bundler wants you to specify rubygems explictly, and prefers that you specify
the https version
for convenience install a puppet gem

Having to export PUPPET_GEM_VERSION all the time is a small headache with
command-line use
add a test for creating subclassed instances
Fix the sample usage in the README

If you follow the example in the README you may not have implemented the
:key_value_separator parameter in your type, and you get the wonderful failure
case:

   Puppet::Error: Invalid parameter key_val_separator(:key_val_separator)

This change looks first, and if the user hasn't specified that attribute in
their type in their type, it falls back to '='
actually write a file in spec/fixtures/tmp

This is useful for testing with puppet apply
allow for the namevar construction

The default type would always have a section as part of its namevar, but if
you're inheriting from ini_setting you may be modelling a flat file with no
sections.

This pushes the formation of the namevar from the section_name and setting into
a method, then demonstrates overriding it so that inherited_ini_setting can just
be

   inherited_ini_file { 'setting':
     value => '12',
   }

and continue to be purgable.