Update to support config of headers sent in requests (issue #125)

[patrick-rodgers]

Q	A
Bug fix?	Yes
New feature?	Yes
New sample?	No
Related issues?	fixes #125

What's in this Pull Request?

This PR contains an update to provide global configuration of the headers sent with each REST request. This resolves #125 by allowing a developer to set "application/json; odata=verbose" for the accept header as shown below:

    pnp.setup({
        headers: {
            "Accept": "application/json; odata=verbose"
        }
    });

    pnp.sp.web.get().then(show);

This also enables the scenario of setting an authentication header using a previously retrieved bearer token - or any other headers that may be needed for a given solution. Currently "headers" is the only configurable property but we will add additional configuration options as they become required or found to provide some benefit.

[hariberry]

In the current version it is:

pnp.setup({
    sp: {
        headers: {
            "Accept": "application/json; odata=verbose"
        }
    }
});