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
...and I just noticed we already have a utility method for that: org.bitcoinj.core.Utils.bigIntegerToBytes(BigInteger, int)... let's use that.