Make the constructors of Array and SharedArray consistent

[polarker]

While Array supports these kind of constructors: Array{Int}(100), SharedArray does not have.

[timholy]

Just an FYI: this and the next two are just for getting rid of the splatting penalty. For a SharedArray, there's sufficient overhead for creation that this is probably negligible, so these probably aren't really necessary. However, neither do they hurt anything.

[polarker]

My original intention was to make ShareArray have the same interface as Array, since I am writing a library using both with type parameters. My fix is kind of naive. And Kevin Squire said in the user group that there could be deeper and cleaner than falling back to SharedArray(T, x), maybe someone could improve later.

[timholy]

Looks nice, and thanks for the contribution!

Could I ask you to add tests for the new constructors? That will reduce the likelihood that any future changes will break your new addition.