Add `bitcoin-php` library for BTC address validation. Use it.

git-svn-id: https://plugins.svn.wordpress.org/my-two-cents/trunk@996272 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update documentation, translation file. Version bump to 0.1.1.

git-svn-id: https://plugins.svn.wordpress.org/my-two-cents/trunk@996273 b8457f37-d9ea-0310-8a92-e5e31aec5664