Specify the escript executable for hooks

Windows appears to not enjoy relative path names for exectuables so
hopefully this fixes things there.

Fixes #83

rebar.config

@@ -36,5 +36,5 @@
     verbose
 ]}.
 
-{pre_hooks, [{"", compile, "./enc compile"}]}.
-{post_hooks, [{"", clean, "./enc clean"}]}.
+{pre_hooks, [{"", compile, "escript enc compile"}]}.
+{post_hooks, [{"", clean, "escript enc clean"}]}.