use interpreted text role to export subscript and superscript (fix #1)

README.org

@@ -340,6 +340,20 @@
   ,**bold** *italic* underlined ``code`` ``verbatim`` strike-through
 #+END_SRC
 
+** Subscript and superscript
+
+   Org:
+#+BEGIN_EXAMPLE
+  H_{2}O
+  E = mc^2
+#+END_EXAMPLE
+
+   reStructuredText:
+#+BEGIN_SRC rst
+  H\ :sub:`2`\ O
+  E = mc\ :sup:`2`\ 
+#+END_SRC
+
 ** Horizontal rules
 
    Org:

ox-rst.el

@@ -1317,9 +1317,7 @@ holding contextual information."
   "Transcode a SUBSCRIPT object from Org to reStructuredText.
 CONTENTS is the contents of the object.  INFO is a plist holding
 contextual information."
-  (if (org-element-property :use-brackets-p subscript)
-      (format "_{%s}" contents)
-    (format "_%s" contents)))
+  (format "\\ :sub:`%s`\\ " contents))
 
 
 ;;;; Superscript
@@ -1328,9 +1326,7 @@ contextual information."
   "Transcode a SUPERSCRIPT object from Org to reStructuredText.
 CONTENTS is the contents of the object.  INFO is a plist holding
 contextual information."
-  (if (org-element-property :use-brackets-p superscript)
-      (format "_{%s}" contents)
-    (format "_%s" contents)))
+  (format "\\ :sup:`%s`\\ " contents))
 
 
 ;;;; Table