tweak readme

[ralphtheninja]

• use level in examples rather than levelup for the following reasons:
  • the install instructions says to install level
  • it's the recommended way of using LevelDB
  • levelup api has changed and accepts an abstract-leveldown compliant store as first argument, i.e. no longer a string
• single quotes everywhere
• align variables (or don't align, I just picked one of them to be consistent)

[coveralls]

Coverage remained the same at 89.339% when pulling 46a87820052aca19703a3c3ebb806b7f3b9c832d on ralphtheninja:master into 17690b27cdc5884cdc3418dd2d11566665af8169 on levelgraph:master.

[ralphtheninja]

@mcollina Merge? :smile:

[mcollina]

done :D