added null device

[flying-sheep]

circumvents #336 and friends

note that the device is still servicable using e.g. dev.new('filename.png'), as i used the null file as default argument.

[flying-sheep]

@JanSchulz:

could you test if plot(main = '\u2014')

1. creates a warning (like the one mentioned at conversion failure #336) and doesn’t create a file without the PR
2. doesn’t create a warning and doesn’t create a file with the PR

on windows?

on linux it behaves as it should

[flying-sheep]

maybe we can add a unit test for this. ideas?

[jankatins]

If so, we nedd to do it in the python test code...

[jankatins]

Mac OS X is included here?

[flying-sheep]

> help(.Platform)
OS.type: character string, giving the *O*perating *S*ystem (family) of
         the computer.  One of ‘"unix"’ or ‘"windows"’.

[jankatins]

init_null_device : null_device: no visible global function definition
  for ‘png’
init_null_device: no visible binding for global variable ‘pdf’
Undefined global functions or variables:
  pdf png
Consider adding
  importFrom("grDevices", "pdf", "png")
to your NAMESPACE file.

[flying-sheep]

fixed, sorry