implement API for certificate generation

[simevo]

this is required by all CMS plugins / framework bundles if they want to be dynamically configurable and do without a preconfguration step on the command line, see for example: simevo/spid-wordpress#22

two new public member functions are required in the Italia\Spid\Sp class:

• isConfigured() returns true if the SP certificates are found where the $settings says they are, and they are valid (i.e. the plugin has been configured correctly)
• generateCerts($ountryName, stateName, localityName, commonName, emailAddress): generates with openssl the certificates where the $settings says they should be (assuming the user which the wenservice is impersonating has write access to that directory)

[simevo]

see PR #66