Closes #474. Added .onUnload method

This ensures datatable.so is properly unloaded on 'detach'.

R/onLoad.R

@@ -82,5 +82,8 @@ getRversion = function(...) stop("Reminder to data.table developers: don't use g
 # 3) The discipline of adding a feaure test here helps fully understand the change.
 # 4) Defining getRversion with a stop() here helps prevent new switches on getRversion() being added in future. Easily circumvented but the point is to issue the message above.
 
-
-
+.onUnload <- function(libpath) {
+    # fix for #474. the shared object name is different from package name
+    # So 'detach' doesn't find datatable.so, as it looks by default for data.table.so
+    library.dynam.unload("datatable", libpath)
+}

README.md

@@ -291,6 +291,8 @@ We moved from R-Forge to GitHub on 9 June 2014, including history.
 
   8.  Added tests (1351.1 and 1351.2) to catch any future regressions on particular case of binary search based subset reported [here](http://stackoverflow.com/q/24729001/559784) on SO. Thanks to Scott for the post. The regression was contained to v1.9.2 AFAICT. Closes [#734](https://github.com/Rdatatable/data.table/issues/704).
 
+  9.  Added a .onUnload method to unload data.table's shared object properly. Since the name of the shared object is 'datatable.so' and not 'data.table.so', 'detach' fails to unload correctly. This was the reason for the issue reported [here](http://stackoverflow.com/questions/23498804/load-detach-re-load-anomaly) on SO. Closes #474. Thanks to Matthew Plourde for reporting.
+
 ---
 
 ### Changes in v1.9.2 (on CRAN 27 Feb 2014)