...any doubts concerning the scope of arbitrable issues should be resolved in favor of arbitration... ...unless it may be said with positive assurance that the arbitration clause is not susceptible of an interpretation that covers the asserted dispute... Section 2 is a congressional declaration of a liberal federal policy favoring arbitration agreements, notwithstanding any state substantive or procedural policies to the contrary. The effect of the section is to create a body of federal substantive law of arbitrability, applicable to any arbitration agreement within the coverage of the act.