Added PBKDF2 (300000 iterations) hashing support in addition to SHA256

Most passphrases don't tend to be very secure or high entropy so I added PBKDF2 support to make the hashes less prone to attacks since they are more expensive to guess. By default it's still using SHA256 but you can now select "Use Secure" to toggle the secure hash mode which uses PBKDF2 with 300000 iterations. Right now using Stanford's sjcl library since that's faster than CryptoJS. Still need to download the JS file and cache it in the repository like the other JS files instead of direct reference to external server.

Also added a button to hide the passphrase although the private keys are still visible. Maybe when we're in hiding mode should hide all private keys?