gtsam-3.1.0 fails to compile on OSX due to forward declared template argument

##Issue

gtsam-3.1.0 fails to compile on OSX (AppleClang 5.1.0.5030040, boost 1.55.0) with the error:

boost/ptr_container/detail/reversible_ptr_container.hpp:104:38: error: base class has incomplete type class reversible_ptr_container : CloneAllocator

##Cause

Values.h declares a typedef KeyValueMap using a forward declared ValueCloneAllocator. However, reversible_ptr_container requires access to a default constructor through inheritance. The declaration follows:

boost/ptr_container/detail/reversible_ptr_container.hpp:

class reversible_ptr_container : CloneAllocator

As this is a template argument, forward declaration does not suffice and an explicit constructor must be provided prior to the template declaration.

##Fix

If forward declaration is preferred to keep the implementation in Values-inl.h, then a default constructor must be explicitly defined. The motivation of the forward declaration of ValueCloneAllocator was unclear to me so I moved it into Values.h.

The attached patch can be applied directly to the release/3.1.0 branch.

##Secondary Issue

The above fix addresses the template call error but does not address the fact that:

reversible_ptr_container()

requires public access to the default constructor. The attached patch also removes the private status for the constructor.

##Validation

Running the test suite on the patched system is successful except for those tests that typically fail with Clang.

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