Improvements to the Collection class

 - Adds default values to various class attributes
 - Makes error messages more verbose
 - Fixes bug in Collection::getRequest()
 - Adds unit tests
Adds classes for Folder and Request attributes.
Adds strict PHPUnit version to phpunit.xml.dist file.
Adds type hint to PostmanItemCollection Interface.
Improvements to the Item class

 - Moves abstract Item class into Items Namespace
 - Naming Convention Amends
Merge branch 'master' of github.com:potherca/PostmanParser