feat(Entity): Add support for string or number type for entity ID

[brandonroberts]

Closes #398

[coveralls]

Coverage remained the same at 91.264% when pulling f52e88c on entity-id-types into e63720e on master.

[coveralls]

Coverage remained the same at 91.413% when pulling fcee651 on entity-id-types into ad30d40 on master.

[Lycidas0815]

Stupid question, since my project broke due to that change: how to I specify the type argument to use properly? My selectId is of type number, but I get a the type argument for type parameter Result cannot be inferred from the usage error in my reducers, using the ngrx-example app as template.

I also had a look into the entity docs but didn't find anything helpful there. The example app changed to type string, but I still want to use my numbers.

Many thanks,
Ly