TLS min/max version updates

Switch to allow `tls_min_version="tls1.0"` from a config file instead of opaque integer (and same for manually setting Config objects).

Default min version tls1.0 to avoid allowing ssl3.0 by default, and specify max TLS version of 1.2 to take advantage of TLS_FALLBACK_SCSV prior to Go 1.5

for more discussion see: #513
Yeah i just ran into issues resolving int here, and string in config file. One option might be to extend  go-options or borrow some approaches we used in go-nsq for reader options to give us a spot to inject translation logic and validation.