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
right, basically the way it works now is that if the config file value is a `string` it gets coerced to the equivalent destination `int` by `go-options`, which is why we *do* need the block you added to use `Set`.

I think it would "just work" if you removed all the changes related to this being a `string`...