Rewrite acccounts and registration.

Save accounts to:

    /etc/letsencrypt/accounts/www.letsencrypt-dmeo.org/acme/new-reg/ \
    kuba.le.wtf@2015-07-04T14:04:10Z/ \
    {regr.json,meta.json,private_key.json}

Account now represents a combination of private key, Registration
Resource and client account metadata. `Account.id` based on the
account metadata (creation host and datetime). UI interface
(`cli._determine_account`) based on the `id`, and not on email as
previously.

Add `AccountStorage` interface and `AccountFileStorage`,
`AccountMemoryStorage` implementations (latter, in-memory, useful for
testing).

Create Account only after Registration Resource is received
(`register()` returns `Account`).

Allow `client.Client(..., acme=acme, ...)`: API client might reuse
acme.client.Client as returned by `register()`.

Move report_new_account to letsencrypt.account, client.Client.register
into client.register.

Use Registration.from_data acme API.

achallenges.AChallenge.key is now the `acme.jose.JWK`, not
`le_util.Key`. Plugins have to export PEM/DER as necessary
(c.f. `letsencrypt.plugins.common.Dvsni.get_key_path`)

Add --agree-tos, save --agree-eula to "args.eula". Prompt for EULA as
soon as client is launched, add prompt for TOS.

Remove unnecessary letsencrypt.network. Remove, now irrelevant,
`IConfig.account_keys_dir`.

Based on the draft from
https://github.com/letsencrypt/letsencrypt/pull/362#issuecomment-97946817.

letsencrypt/plugins/common.py

@@ -4,6 +4,7 @@ import pkg_resources
 import shutil
 import tempfile
 
+from cryptography.hazmat.primitives import serialization
 import zope.interface
 
 from acme.jose import util as jose_util
@@ -157,6 +158,11 @@ class Dvsni(object):
         return os.path.join(
             self.configurator.config.work_dir, achall.nonce_domain + ".crt")
 
+    def get_key_path(self, achall):
+        """Get standardized path to challenge key."""
+        return os.path.join(
+            self.configurator.config.work_dir, achall.nonce_domain + '.pem')
+
     def _setup_challenge_cert(self, achall, s=None):
         # pylint: disable=invalid-name
         """Generate and write out challenge certificate."""
@@ -170,6 +176,16 @@ class Dvsni(object):
         with open(cert_path, "w") as cert_chall_fd:
             cert_chall_fd.write(cert_pem)
 
+        key_path = self.get_key_path(achall)
+        key_pem = achall.key.key.private_bytes(
+            encoding=serialization.Encoding.PEM,
+            format=serialization.PrivateFormat.TraditionalOpenSSL,
+            encryption_algorithm=serialization.NoEncryption())
+        # XXX: key_file chmods!  (SEC)
+        with open(key_path, 'w') as key_file:
+            key_file.write(key_pem)
+        self.configurator.reverter.register_file_creation(True, key_path)
+
         return response