Avoid CompressionError for newlines in stderr

UglifyJS2 prints two newlines '\n\n' into stderr for me.
The `if stderr` in the `SubProcessCompressor.execute_command` raises a CompressionError if that happens.
I've added a `strip()` call to avoid that.
Avoid CompilerError for newlines in stderr.
Merge pull request #184 from fabianbuechler/patch-1

avoid errors for newlines in stderr