Sslcerts

Allow connecting to etcd in SSL mode (Master already used `ssl_*` params etc but was throwing `OpenSSL::SSL::SSLError` because certs were not loaded and just provided as file paths).

why you want this inside the library? this as well as private key can be injected from outside as well right? the client in its current form allow this.
Reason im asking this, we have to do error handling around file reads if we include this patch. also, this is bit unconventional, i have not seen any http wrapper gem that allows such parameters