1/2 use forked spongycastle to speed up secret key imports
2/2 use forked spongycastle to speed up secret key imports
enable passphrase changing for subkey only keychains
Merge pull request #51 from ashh87/master

Updates spongycastle to the fork:
https://github.com/ashh87/spongycastle

This allows us to import secret keys much more quickly and to enable passphrase changing for subkey only secret keys.