Add getLatestHeader() and getLatestBlock() methods, fix saveHeads() bug and update API docs

[vpulim]

• Added new methods to retrieve the latest header and full block
• Fixed a bug that prevented the latest header and block from being saved to the db
• Made some API doc changes to help clarify the difference between iterator heads and latest head/block. This hopefully helps explain the difference between the getHead(), getLatestHeader() and getLatestBlock() methods.

[coveralls]

Coverage increased (+0.08%) to 96.84% when pulling fe0c8e8 on vpulim:fix-head-saving into c4fca13 on ethereumjs:master.

[holgerd77]

This looks good, thanks for the immediate PR submission, great!

Also tested this locally with my custom state-keeping test file using a leveldown DB, can confirm that this works.