documentation improvements; system.nim docs adapted to unix's braindead idea of how software should be packaged which didn't work in 1970 and surprise doesn't work in 2015 either

doc/tut2.txt

@@ -215,7 +215,7 @@ postfix notation.)
 So "pure object oriented" code is easy to write:
 
 .. code-block:: nim
-  import strutils
+  import strutils, sequtils
 
   stdout.writeLine("Give a list of numbers (separated by spaces): ")
   stdout.write(stdin.readLine.split.map(parseInt).max.`$`)

lib/system.nim

@@ -14,14 +14,6 @@
 ## explicitly. Because of this there cannot be a user-defined module named
 ## ``system``.
 ##
-## Exception hierarchy
-## ===================
-##
-## For visual convenience here is the exception inheritance hierarchy
-## represented as a tree:
-##
-## .. include:: ../doc/exception_hierarchy_fragment.txt
-##
 ## Module system
 ## =============
 ##