Contributions from DDvO

These are contributions from DDvO.  Most of the changes are to the example client/server code.  There are a couple of changes to libest itself.  Here's a summary of the changes:

1) createca.sh script now uses sha-256 instead of sha-1
2) added details to README in example/client directory
3) Added -x and -y options to estclient to allow for externally generated CSR
4) Removed hard-coded "estclient_keypair" in estclient
5) Fixed verbose output in example client/server code
6) Added -o option to estserver, which enables the new operational mode for HTTP auth.  
7) Added new operational mode for HTTP auth when in server or proxy mode.  Under this mode HTTP authentication is only performed when the TLS client auth fails.  This is enabled by using the new est_set_http_auth_required() entry point.
8) Added EST_ERR_NO_CERT error code.  Fixed several places where EST_ERR_NO_CSR was returned by the API when in reality a certificate wasn't available.
9) Modified est_client_enroll_csr() to no longer require a private key.  This allows for an externally generated CSR to be provided without requiring a private key, however PoP will no be possible under this scenario.