Initial DNS Resolvers and SPF scaffolding (#123)

* Implemented Live and Preloaded resolvers

* Integrated Craig's parser.

dnsresolver/dnscache.go

@@ -0,0 +1,28 @@
+package dnsresolver
+
+// dnsCache implements a very simple DNS cache.
+// It caches the entire answer (i.e. all TXT records), filtering
+// out the non-SPF answers is done at a higher layer.
+// At this time the only rtype is "TXT". Eventually we'll need
+// to cache A/AAAA/CNAME records to to CNAME flattening.
+type dnsCache map[string]map[string][]string // map[fqdn]map[rtype] -> answers
+
+func (c dnsCache) get(label, rtype string) ([]string, bool) {
+	v1, ok := c[label]
+	if !ok {
+		return nil, false
+	}
+	v2, ok := v1[rtype]
+	if !ok {
+		return nil, false
+	}
+	return v2, true
+}
+
+func (c dnsCache) put(label, rtype string, answers []string) {
+	_, ok := c[label]
+	if !ok {
+		c[label] = make(map[string][]string)
+	}
+	c[label][rtype] = answers
+}