Use maven. Change folder structure. Remove responsabilities

Hi, I moved to a maven POM schema and changed the folder structure.

Sorry it's a big commit, I had no way of doing it partially. What should have been another pull request is the modifications I did to BitPay.java and KeyUtils.java, where I removed some default behaviour in order to allow the user more control over how should the files be loaded. Tests are broken and we should try to get them working (I'll try to do it tomorrow).

Let me know if there's anything that you'd like me to change.

If you're using eclipse, you can use "Maven > Import Java project from pom.xml file"