chore(build): build using webpack

BREAKING CHANGE: we now distribute the library in two different ways:

* the npm "main" is now `lib/index.js`, which is ES5 with require statements, compatible with webpack and browserify
* the `dist/payment.js` is a pre-built JS file that can be included in <script> tags

.gitignore

@@ -1 +1,4 @@
+.tmp
+coverage
 node_modules
+lib

.npmignore

@@ -0,0 +1,52 @@
+# Automatically ignored per:
+# https://www.npmjs.org/doc/developers.html#Keeping-files-out-of-your-package
+#
+# .*.swp
+# ._*
+# .DS_Store
+# .git
+# .hg
+# .lock-wscript
+# .svn
+# .wafpickle-*
+# CVS
+# npm-debug.log
+# node_modules
+
+*.seed
+*.log
+*.csv
+*.dat
+*.out
+*.pid
+*.gz
+*.orig
+*.jql.js
+
+work
+build
+src
+test
+spec
+pids
+logs
+results
+coverage
+lib-cov
+html-report
+xunit.xml
+
+.project
+.idea
+.settings
+.iml
+*.sublime-workspace
+*.sublime-project
+
+ehthumbs.db
+Icon?
+Thumbs.db
+.AppleDouble
+.LSOverride
+.Spotlight-V100
+.Trashes