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
...but it doesn't hurt to present exactly 32 bytes (256 bits) either, especially if
1) at least one provider stumbles over 33 bytes and
2) we already have the code for it, since 2011.

To make this clear, imho all we need to do is use
Utils.HEX.encode(getPrivKeyBytes())
instead of
Utils.HEX.encode(getPrivKey().toByteArray())