Allow for a custom HttpMessageHandler to be provided to the internal HttpClient instance

[bobstanton]

Gathering tracing metrics for an application's Http requests can be done easily with a custom HttpMessageHandler as seen here: http://stackoverflow.com/a/18925296

In order to get this behavior with the current version of the Box SDK, an entire HttpRequestHandler class needs to be provided to the BoxClient. A lighter weight alternative where by a HttpMessageHandler can be provided is preferred.

[ghost]

Allowing for a custom HttpMessageHandler would also benefit testability: https://github.com/richardszalay/mockhttp#how

[mattwiller]

@bobstanton @Nyxel We'll add it to our backlog to investigate!