updated toString in ECKey, added helper methods and tests

Based on this discussion: https://groups.google.com/forum/?fromgroups=#!topic/bitcoinj/QxLpodGLNhw

I have updated toString method in ECKey to include WIF formats in the output. It assumes that we want this on main network - not sure how to parametrize that right now
Ah I just found this quote from @mikehearn : "The reason for the leading zero byte is that the private key is a number stored as a BigInteger. The zero byte disambiguates it from being negative. If blockchain.info can't accept a private key written in this way for some reason, I think the best bet would be to just remove the leading zero byte before import. What bitcoinj is doing here is not technically wrong, as far as I am concerned, though we can fix it without too much bother."