Refactor the core of the lib (#700)

- Packages
- Isolate code used by the CLI into the package `cmd`
- (experimental) Add e2e tests for HTTP01, TLS-ALPN-01 and DNS-01, use [Pebble](https://github.com/letsencrypt/pebble) and [challtestsrv](https://github.com/letsencrypt/boulder/tree/master/test/challtestsrv) 
- Support non-ascii domain name (punnycode)
- Check all challenges in a predictable order
- No more global exported variables
- Archive revoked certificates
- Fixes revocation for subdomains and non-ascii domains
- Disable pending authorizations
- use pointer for RemoteError/ProblemDetails
- Poll authz URL instead of challenge URL
- The ability for a DNS provider to solve the challenge sequentially
- Check all nameservers in a predictable order
- Option to disable the complete propagation Requirement
- CLI, support for renew with CSR
- CLI, add SAN on renew
- Add command to list certificates.
- Logs every iteration of waiting for the propagation
- update DNSimple client
- update github.com/miekg/dns

acme/api/service.go

@@ -0,0 +1,45 @@
+package api
+
+import (
+	"net/http"
+	"regexp"
+)
+
+type service struct {
+	core *Core
+}
+
+// getLink get a rel into the Link header
+func getLink(header http.Header, rel string) string {
+	var linkExpr = regexp.MustCompile(`<(.+?)>;\s*rel="(.+?)"`)
+
+	for _, link := range header["Link"] {
+		for _, m := range linkExpr.FindAllStringSubmatch(link, -1) {
+			if len(m) != 3 {
+				continue
+			}
+			if m[2] == rel {
+				return m[1]
+			}
+		}
+	}
+	return ""
+}
+
+// getLocation get the value of the header Location
+func getLocation(resp *http.Response) string {
+	if resp == nil {
+		return ""
+	}
+
+	return resp.Header.Get("Location")
+}
+
+// getRetryAfter get the value of the header Retry-After
+func getRetryAfter(resp *http.Response) string {
+	if resp == nil {
+		return ""
+	}
+
+	return resp.Header.Get("Retry-After")
+}