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
What if we in getPrivKeyAsHex check for the case with "first byte = 0 and length = 33" and in this case just print the last 32 bytes, but still use Utils.HEX for it?