Add credentials for TileWMS

[MTD-Thomas]

Add a new property on Layer called Credentials to set the header Authorization during tile calls

In interop Javascript, only for TileWMS, I set tileLoadFunction to use these credentials if defined

Closes #105

[lolochristen]

Thanks a lot for you PR. I like your approach. I will review asap with some comments/suggestions.

[MTD-Thomas]

I will probably need later a different way to transfer credentials, right now I only managed Basic authentication but later I will probably need to implement Bearer.

So when the time comes I will improve this code

[MTD-Thomas]

We probably could merge every cases by creating a property AuthorizationHeader instead of Credentials which will contains both Bearer or Basic followed by the value and use this whole property to define the header in the query

Tell me if you want me to change my code