Git hygiene

Marked .DS_Store files as ignored
add basic code generation testing rig and 1st test.
Fixes #554
Merge pull request #566 from parrt/args-as-templates

Args as templates
Add regression test for #561 (token named ATN breaks reference to ATN.INVALID_ALT_NUMBER)
Qualify references to the ATN class to avoid symbol conflicts with tokens named ATN (fixes #561)
Merge pull request #575 from sharwell/fix-561

Qualify references to ATN class in generated code if it would alias a token