Add lintr to testthat tests

[nuest]

See https://github.com/jimhester/lintr#testthat

• [ ] lint runs within a Travis Build

[MatthiasHinz]

Lint should run after successful build, but Travis fails at the moment :( .

I did not include it in testthat, because it throws many style-issues and causes R CMD Check to fail (it does not terminate).

lintr many need to be customized for messages like

'lines should not be more that 80 characters'

[nuest]

We can add a file .lintr to configure what is checked, see https://github.com/jimhester/lintr#project-configuration