Export safeCall function directly

The current API is awkard for error checking:
    var safeCall = require('safetydance').safeCall;
    safeCall(function () { ... });
    // oops, no way to check error because error is part of safetydance module

Alternately,
    var safe = require('safetydance');
    safe.safeCall(function () { ... }); // slightly ugly
    safe.error; // contains the error

With this change:
    var safeCall = require('safetydance');
    safeCall(function () { ... });
    safeCall.error; // contains the error