Polish up examples (somewhat)

- Mention them in the docs (arguably a bit hamfistedly).
- Make the README an RST.
- Make them pass flake8 and add flake8 to tox.ini

They should all be rewritten and made Python 3-friendly but that's out
of scope here.
diff --git a/examples/certgen.py b/examples/certgen.py
index da0624f..7b70e98 100644
--- a/examples/certgen.py
+++ b/examples/certgen.py
@@ -13,6 +13,7 @@
 TYPE_RSA = crypto.TYPE_RSA
 TYPE_DSA = crypto.TYPE_DSA
 
+
 def createKeyPair(type, bits):
     """
     Create a public/private key pair.
@@ -25,6 +26,7 @@
     pkey.generate_key(type, bits)
     return pkey
 
+
 def createCertRequest(pkey, digest="sha256", **name):
     """
     Create a certificate request.
@@ -52,7 +54,9 @@
     req.sign(pkey, digest)
     return req
 
-def createCertificate(req, issuerCertKey, serial, validityPeriod, digest="sha256"):
+
+def createCertificate(req, issuerCertKey, serial, validityPeriod,
+                      digest="sha256"):
     """
     Generate a certificate given a certificate request.
 
@@ -78,4 +82,3 @@
     cert.set_pubkey(req.get_pubkey())
     cert.sign(issuerKey, digest)
     return cert
-