Fix PUB/SUB topics support

[drr00t]

Hi, PR to fix #31 issue, we need expose internal constants in a better way, i don't like this.

var nn = nano._bindings;
sub.setsockopt(nn.NN_SUB, nn.NN_SUB_SUBSCRIBE, 'my.topic');

thank you for bind Nick.

[nickdesaulniers]

Probably don't need a switch block when a single conditional would suffice?

[nickdesaulniers]

@tcr what are your thoughts on this?

[drr00t]

Hi, sorry about that.

I should have removed it, I was thinking of a way to inform the constants but i didn´t like and i forgot.

My idea is do something like this to expose constants: https://github.com/JustinTulloss/zeromq.node/blob/master/lib/index.js#L93

This would be changed too: https://github.com/nickdesaulniers/node-nanomsg/blob/master/lib/index.js#L19

We won´t need pass enums like strings anymore: https://github.com/nickdesaulniers/node-nanomsg/blob/master/lib/index.js#L19

[asafyish]

+1

[nickdesaulniers]

completed in PR: #65, will publish updated version soon!