readme: cleanup
readme: cleanup

- stylize levelup
- remove windows section; covered in leveldown's readme.
- simplify ctor docs
- clarify events
  - link to EventEmitter
  - put events in a table
  - indicate timeframe: "has been deleted" instead of "is deleted"
  - it's key-driven, so say "key" instead of "value has been deleted"
  - prefer "batch has executed" over "batch operation has executed", which
    seemed to suggest this event fires for each operation within a batch.
  - reorder open-state events naturally
  - add short example
- mention awesome next to wiki
- remove level-sublevel
  - we should test it with v2 and then maybe add it back in.
  - subleveldown may be a better alternative, but this is not yet
    compatible with levelup v2
  - awesome is the better place for stuff like this anyway
- remove short description, merge into intro

Level/levelup