Netty Headers Class Restructure and Algorithm Updates

Motivation:
Headers within netty do not cleanly share a common class hierarchy.  As a result some header types support some operations
and don't support others.  The consolidation of the class hierarchy will allow for maintenance and scalability for new codecs.
The existing hierarchy also has a few short comings such as it is not clear when data conversions are happening.  This
could result unintentionally getting back a collection or iterator where a conversion on each entry must happen.

The current headers algorithm also prepends all elements which means to find the first element or return a collection
in insertion order often requires a complete traversal followed by a collections.reverse call.

Modifications:
-Provide a generic base class which provides all the implementation for headers in netty
-Provide an extension to this class which allows for name type conversions to happen (to accommodate legacy CharSequence to String conversions)
-Update the headers interface to clarify when conversions will happen.
-Update the headers data structure so that appends are done to avoid unnecessary iteration or collection reversal.

Result:
-More unified class hierarchy for headers in netty
-Improved headers data structure and algorithms
-headers API more clearly identify when conversions are required.
@nmittler - I think that is a good idea.  I guess what I'm wondering though is using the array directly OK?  I can't remember the exact scenario that I was in but I was checking if `hasArray()` was true and used the `array()` method...but not all the data was present in the array.  I vaguely remember `CompositeByteBuf` being the actual buffer that was used.

So the question is does the `hasArray()` method guarantee that all the data will be in the array returned by `array()`, or is there a situation where `hasArray()` is true but you need to do a `writeBytes()` into a temporary buffer allocated to the size of `readableBytes()`?