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
Oh i see what you are saying; i added these in a different order. the validation i added in Main can coerce to int before it's merged into `nsqOptions`. Not as elegant as a generic support in `go-options` but perhaps sufficient.