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
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.
fix syntax for 1.8.7
.instances isn't stable between puppet versions

Extract the names from all the instances, and check they don't have the form
`[ '/green', '/red' ]` which they would if the inheritance doesn't work.
Merge pull request #50 from richardc/bug/inherited_purging

Bug/inherited purging