Add IDEA files to gitignore

Required to prevent NPM from including these files in the package.

Author: tocker

.gitignore

@@ -25,3 +25,9 @@ build/Release
 # Dependency directory
 # https://www.npmjs.org/doc/misc/npm-faq.html#should-i-check-my-node_modules-folder-into-git
 node_modules
+
+# Covers JetBrains IDEs: IntelliJ, RubyMine, PhpStorm, AppCode, PyCharm
+*.iml
+
+## Directory-based project format:
+.idea/