Fix showItemInFolder not work because of relative path 

see #913.
Linux: at last it will call `execvp(argv_cstr[0], argv_cstr.get());`
if the argv is relative path, it opens the file relative to the place, where it is launched

Mac:at last it will call `selectFile`.According to [this](https://developer.apple.com/library/mac/#documentation/cocoa/reference/ApplicationKit/Classes/NSWorkspace_Class/Reference/Reference.html), `[[NSWorkspace sharedWorkspace] selectFile` only receive fullPath 

windows: When it call `desktop->ParseDisplayName`.According to [this](http://msdn.microsoft.com/en-us/library/windows/desktop/bb775090)
```ParseDisplayName is not expected to handle the relative path or parent folder 
indicators(".\" or "..\"). It is up to the caller to remove these appropriately.```

so I change the relative path to full path