fixed setAutocompactionInterval interval arg

if interval is undefined, then it's bigger than 5000

Test case in Chrome console:
```
> var interval
undefined
> (interval < 5000)
false
```
Merge pull request #141 from b1rdex/patch-1

fixed setAutocompactionInterval interval arg
Some styling
Update README.md
Merge branch 'master' of https://github.com/louischatriot/nedb