Support union types

[jgiles]

Example:

/** @type {(MyClass|YourClass)} */
var union;
union; //: (MyClass|YourClass)

More importantly, we want proper autocompletion of properties on all types in the union.

[MetaMemoryT]

I have created 2 pull requests: #34 ternjs/tern#405

[angelozerr]

tern 0.13.0 should support now this issue.