Fixes #20
instance of now also supports interface
Moved some handy code up the tree
Removed todo to look into a OR statement for valid key. Not done due to the fact it will lead into a recursive call aka never ending loop
Minor code formatting

AbstractTypeAssertion extends the AbstractValueAssertion
Implement the `ValueAssertionInterface`
Closes #16
Changed the messages
Added the "SYSTEM REQUIREMENTS" to the readme
Closes #14
Changed phpDoc signature
Changed signature
Code format