Expose Structure via `require('immstruct/structure');`
Better hasIn

Based on https://github.com/facebook/immutable-js/commit/8139d425db04ef93c31054c18439929119be2e79
Merge pull request #11 from Dashed/patch-1

Exposes structure as public API.
Adds documentation for creating a manual instance of Structure
Use immutablejs hasIn implementation whenever it's available.
Merge pull request #14 from Dashed/better-hasIn

Improves hasIn. Making it future proof with Immutable.js implementation
Renames randString to better reflect intention.