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
Utils.HEX ist the Guava BaseEncodings.base16() encoding, which implements http://tools.ietf.org/html/rfc4648#section-8

Can you try to find the reason for leading 00 in your linked example? Maybe getPrivKey().toByteArray() just *has* this one zero prepended? @mikehearn , do you know if private keys can be 33 bytes in length or are they always supposed to have 32 bytes?