[0.10.2] fix importer and add unit tests

[Severino]

• Importer now automatically removes BOM if present
• Better readable format for error message on validation
• Renamed fromImport to parseImport on the attribute classses. The base class now by default imports the passed string, removing redundancies on the string-based classes.
• More elaborate type checking on attributes parseImport method.
• Unit test for all attribute imports (except Table due to complexity)

Run all tests using php artisan test .\tests\Unit\Attributes