Adds test for browserify

This commit adds tests to make sure that the browserify bundle will work in the dist file of clipboard.js

This commit adds the mocha and chai modules, since karma doesn't work well with node only tests.

Also splited tests tasks in package.json and updated .gitignore

test/node.js

@@ -0,0 +1,11 @@
+var assert = require('chai').assert;
+var browserify = require('browserify');
+
+describe('Node', function() {
+    it('should import the lib in a commonjs env without babel', function(done) {
+        browserify('./dist/clipboard.js').bundle(function(err) {
+            assert.equal(err, null);
+            done();
+        });
+    });
+});