Fix negative number syntax. Add a unary '-' operator.

Closes #63.

testdata

@@ -21,6 +21,11 @@ null
 null
 1
 
+
+-1
+null
+-1
+
 # FIXME: much more number testing needed
 
 {}
@@ -197,6 +202,14 @@ null
 "wtasdf"
 2.0
 
+2-1
+null
+1
+
+2-(-1)
+null
+3
+
 1e+0+0.001e3
 "I wonder what this will be?"
 20e-1