Describe Uncompressed And Compressed Public Keys

Preview: http://dg3.dtrt.org/en/developer-guide#public-key-formats

A quick little subsection the two different public key formats possible in Bitcoin.  Thanks to @sipa for [inspiration on graphing the secp256k1 curve](http://bitcoin.stackexchange.com/questions/21907/what-does-the-curve-used-in-bitcoin-secp256k1-look-like).  (I plan to replicate his nice prime field illustration someday.)

Note: diff went a bit crazy on the reordered base58 private key list.  I just added a new item and reordered the subsequent items.