make DeterministicSeed constructors public

I am experimenting with the HD wallets and looking into the DeterministicSeed class. 

I've assumed a possible usage is 
```java
new DeterministicSeed("mnemonicCode as string", "passphrase", 12345);
```
But so far this is not a public constructor[1] - though passing the mnemonicCode as List<String> is a public constructor [2].

The constructor accepting the code as String simply also does the String decoding/splitting of the word list and I assume it could be marked as public. 

This PR makes the constructors public. 

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[1] https://github.com/bitcoinj/bitcoinj/blob/master/core/src/main/java/com/google/bitcoin/wallet/DeterministicSeed.java#L53
[2] https://github.com/bitcoinj/bitcoinj/blob/master/core/src/main/java/com/google/bitcoin/wallet/DeterministicSeed.java#L78