OkHttp 3

[devemux86]

Update to OkHttp 3.

[andreynovikov]

I can do it but in terms of #140 is it still needed?

[devemux86]

In #140 I intend to separate OkHttp from vtm core, probably with a vtm-http module.
Like I did for JTS external dependency in #53 by creating the vts-jts module.

I won't remove anything from the library, so you can proceed freely with the update (I can work with any result).

[andreynovikov]

Going on trip next week. Will do it in a month. You can assign it to me if you wish.

[devemux86]

I created vtm-http module for the external dependencies like OkHttp.
Moving there also its class, so it can be updated to latest version.

[devemux86]

I pushed an improved API for OkHttpEngine which permits external OkHttpClient.Builder, allowing for a wide range of customizations for OkHttp client used in VTM.