Adds internet explorer utility

Adds a utility for checking any version of Internet Explorer.
Adds origin setting

This adds a "origin" setting that is derived from the "baseURL". The need for
this setting came from the requirement to validate the origin of messages
sent via window.postMessage.
Refactor paypal integration

This refactors the paypal integration to deal with various bugs in all
versions on Internet Explorer.
Rebuild src
Merge pull request #101 from recurly/relay

Improve paypal integration