Fix problem with Virtus extend module

After including Virtus to a module and then including that module to another class
in particular with Boolean type a const_missing error appears. The solution was to
add the ClassMethods#const_missing method to ModuleExtensions.
Refactor: Remove duplication between ClassMethods module and ModuleExtensions

Add a new Common module to share the const_missing method between the two modules
Refactor: Name of module to share const_missing behavior

ConstMissingExtensions looks like a better name than Common
mend
Fix the loading of const_missing_extensions

common file name doesn't exist anymore the new name of the module and filename is const_missing_extensions
Merge pull request #176 from cored/170-extended-modules-const-missing

Fix problem with Virtus extend module (fixes #170)