subject mail populated when user shares >>file/folder<<

in response to @rperezb issue #767 , the subject mail is now populated when the user shares either a file or folder. I also kept the same syntax as it is on the web, with file/folder name surrounded with greater and less signs. 
Only one minor change more... 
Maybe it is better if we also define this string as a resource in strings.xml file as follows:

<string name="subject_token">%1$s %2$s >>%3$s<< %4$s</string>

In this case, if we need to change it in the future or maybe introduce text that can be translated, it is prepared :)