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
I think this is the correct change (being a `uint16`) - but it means we need to update `go-options` to coerce properly