Add gitignore to github repo.

This is based on an external GitHub commit by JakeWharton.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=81830485
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..fa7042b
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,9 @@
+# Maven
+target
+
+# IntelliJ IDEA
+.idea
+*.iml
+
+# OS X
+.DS_Store