Adding string.h for strdup()

Without this, an error is thrown when compiling : 

```
../src/kexec.cc: In function ‘v8::Handle<v8::Value> kexec(const v8::Arguments&)’:
../src/kexec.cc:83:36: error: ‘strdup’ was not declared in this scope
             argv[i+1] = strdup(*arg);
```

This commit fixes it.
It also stripe whitespaces at new lines.