improve granularity of printed time

although I appreciate @marceloboeira's efforts in #63, I don't like
how imprecise the printed time becomes.

to demonstrate, this is how `ms` converts miliseconds

```javascript
var time = require('ms')

time(2000)    // '2s'
time(62000)   // '1m'
time(3662000) // '1h'
```

and this is how `humanize-time` does it

```javascript
var time = require('humanize-time')

time(2000)    // '2s'
time(62000)   // '1m, 2s'
time(3662000) // '1h, 1m, 2s'
```