Updates missing Spanish keys
Fixes feature request #726.

Created the class "net.sf.jabref.export.OpenFolder.java" that extends the class "net.sf.jabref.AbstractWorker" to capture the selected element and the path to the PDF file;

Created the attribute "ON_LINUX" in class "net.sf.jabref.Globals.java";

Created "FOLDER_FIELD" in class "net.sf.jabref.GUIGlobals.java";

Added attribute "openFolder" to "GeneralAction" in class "net.sf.jabref.JabRefFrame.java";

Added action "openFolder" class "net.sf.jabref.RightClickMenu.java";

Created methods "openFolder (String link)", "openFolderOnLinux (String link)", "openFolderOnWindows (String link)" and "openFolderGeneric (String link)" in class "net.sf.jabref.Util.java";

Added key "Open_folder" in language files "en" and "pt_BR".
Improves openFolders
 * renames method to more speaking names
 * uses File.getParentFile()
 * adds logging if folder openening throws an exception
 * replaces usage of System.out.println() by logger
 * message is now output to frame
 * localizations for other languages added
 * formats code to follow JabRef coding conventions
Send as email now uses Util.openFolderAndSelectedFile
Logging of exceptions in all catches of RightClickMenu.java
Using varargs ("...") in Globals.java
Fixes UTF8 encoding in CHANGELOG
Adds "Open folder" to context menu of file entry.
  * adds missing keys to JabRef_zh.properties
Updates version to 2.10beta