Ensuring that AF::Base.find("") raises exception

Prior to this patch, a new object would be instantiated & returned when
ActiveFedora::Base.find("") was called.

Unrelated specs began breaking due to a change in rdf-rdfxml at version
1.0.2
Merge pull request #156 from projecthydra/fix-active-fedora-find-on-blank

Ensuring that AF::Base.find("") raises exception
class_from_string should find sibiling classes
Merge pull request #158 from projecthydra/improve_class_from_string

class_from_string should find sibiling classes
Bumping version to 6.4.3
The example test should look for Base.exists?() rather than checking to
see that an ObjectNotFound error is raised.

The previous test triggers this deprecation warning:
```
DEPRECATION: `expect { }.not_to raise_error(SpecificErrorClass)` is
deprecated. Use `expect { }.not_to raise_error()` instead. Called from
/Users/.../gems/rspec-expectations-2.14.0/lib/rspec/matchers/built_in/raise_error.rb:53:in
`does_not_match?'.
```
Merge pull request #159 from projecthydra/better_test

The example test should look for Base.exists?()
Updating CONTRIBUTING.md as per Hydra v6.0.0

[ci skip]
Allow query parameter AF.reindex_everything

ActiveFedora::Base.reindex_everything certainly does that. It will
search out all objects from Fedora and attempt to load them into an
ActiveFedora::Base model. If you ran this from an application that has
only a subset of the models in Fedora you could be in for some really
weird surprises.

This patch allows you to pass a Fedora query string to the underlying
Rubydora search method.

In the case of our application, I only want to reindex items that are in
the namespace "und", so my method call looked like the following:

    `ActiveFedora::Base.reindex_everything("pid~und:*")`

Closes #160
Merge pull request #161 from projecthydra/allow_reindex_everything_to_take_a_query_string

Allow query parameter AF.reindex_everything
Adding "An ActiveModel" shared behavior

Providing a shared behavior for verifying that ActiveFedora::Base (and
its descendants) implement the ActiveModel interface as defined by
ActiveModel::Lint::Test.
Merge pull request #162 from projecthydra/adding-lint-validation-to-active-fedora

Adding "An ActiveModel" shared behavior
Don't reindex fedora-system objects in reindex_everything
Merge pull request #163 from projecthydra/update_index_improvement

Don't reindex fedora-system objects in reindex_everything
Add tests for relationships when class_name is 'ActiveFedora::Base' [log skip]
Filters belongs_to associations by :class_name if specified

Added logic for the removal of the correct relationship, when multiple rels exist with the same predicate. Added spec.

Added class matching to belongs_to associations
has_and_belongs_to_many can have more than 10 results.
Merge pull request #167 from projecthydra/habtm_10_results

has_and_belongs_to_many can have more than 10 results.
Use get_config_path to get the path for the predicate_mappings.yml

This simplifies the code for getting this file by leveraging code used
to find the solr.yml and fedora.yml files.  Additionally this fixes a
bug where predicate_mappings.yml is not found in the local rails
application.
Merge pull request #168 from projecthydra/detect_predicate_config

Use get_config_path to get the path for the predicate_mappings.yml