Upgrade to BouncyCastle 1.51 and JUnit 4

BouncyCastle 1.51 was released yesterday. Since BC 1.50 hasn't merged into stable yet, we might as well go straight to BC 1.51. After talking with nextgens and others on IRC we decided that we should move to JUnit 4 as all JUnit 3 tests will still work in 4 and Debain Wheezy has the junit4 package.