add go.mod for use with vgo

[igm]

As vgo became accepted proposal let's add go.mod. Especially for v2, so that import path can be in the form vgo understands, ie;
import "github.com/igm/sockjs-go/v2/sockjs"

[igm]

The steps:

• create a PR with/ v2 and subdirectory in the root tree with the content of branch v2 with adding support for go modules and merge into master
• after the PR merge tag the master with v2.0.2
• create a PR for branch v2 with go modules support by adding go.mod file with module gopkg.in/igm/sockjs-go.v2 and tag branch with a new tag v2.1.0
• create a PR for master with / v3 subdirectory in the root with the content of current master which is a current dev version and soon will be released as v3

[igm]

fixed in v2 and v3