Add .dir-locals and set c-basic-offset to 2.

This makes it easier for emacs users to automatically get the right
2-space indentation when they edit curl source files.

c++-mode is in there as well because Emacs can't easily know if
something is a C or C++ header.

Closes #574

.dir-locals.el

@@ -0,0 +1,10 @@
+;;; Directory Local Variables
+;;; See Info node `(emacs) Directory Variables' for more information.
+
+((nil . ((indent-tabs-mode . nil)
+         (show-trailing-whitespace . t)))
+ (c-mode . ((c-basic-offset . 2)
+            ))
+ (c++-mode . ((c-basic-offset . 2)
+              ))
+ )