Added Opa.gitignore

A .gitignore template for the Opa language <http://opalang.org>

Opa.gitignore

@@ -0,0 +1,13 @@
+_build
+_tracks
+
+opa-debug-js
+
+*.opp
+*.opx
+*.opx.broken
+*.dump
+*.api
+*.api-txt
+*.exe
+*.log