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
Mike spotted you're changing the formatting of private keys from "Utils.HEX.encode(getPrivKey().toByteArray())" to "getPrivKey().toString(16)". What's the reason? On StackExchange I read .toString(16) is stripping leading zeros. I'd personally prefer to at least keep bytes intact.