Resolve #56, provide jss instance via styled

[lttb]

Now we can set jss plugins without the need to create special styled function.
For example:

import styled from 'styled-jss'
import camelCase from 'jss-camel-case'

styled.jss.use(camelCase())

Note that it's better to set up all the plugins once somewhere in one place

[coveralls]

Coverage remained the same at 100.0% when pulling 7fd83fb on feature/provide-jss-instance-in-styled into eec629d on master.