Adds a new mechanism to load providers from any gem or file path. (#263)

* Adds ability to load only providers used in config file

spec/unit/providers/base_spec.rb

@@ -1,4 +1,5 @@
 require 'spec_helper'
+require 'vmpooler/providers/base'
 
 # This spec does not really exercise code paths but is merely used
 # to enforce that certain methods are defined in the base classes

spec/unit/providers/dummy_spec.rb

@@ -1,4 +1,5 @@
 require 'spec_helper'
+require 'vmpooler/providers/dummy'
 
 describe 'Vmpooler::PoolManager::Provider::Dummy' do
   let(:logger) { MockLogger.new }

spec/unit/providers/vsphere_spec.rb

@@ -1,5 +1,6 @@
 require 'spec_helper'
 require 'mock_redis'
+require 'vmpooler/providers/vsphere'
 
 RSpec::Matchers.define :relocation_spec_with_host do |value|
   match { |actual| actual[:spec].host == value }