Support for additional MDCAdapter implementations

[ceki]

At present time, the org.slf4j.MDC class supports a single MDCAdapter implementation.

However, several users have asked to be able add custom MDCAdaper implementations. How should these implementations be supported?

Here is are some possible approaches to this problem:

1. add a new methods such as addMDCAdapter getMDCAdapters in the org.slf4j.MDC class
2. add a new method such as addNextAdapter in MDCAdapter interface to allow chaining of adapters.

Approach 1 offers a relatively simple API but at the cost of exposing implementation specific details at the user level.
Approach 2 hides implementation specific details from the user but at the cost of a slightly more complex API.