GenericValue: accept different allocators for read-only access

Several GenericValue functions take a const-reference to another GenericValue
only.  These functions can safely accept values with a different allocator
type.

The following functions are therefore extended by a SourceAllocator template
parameter in this commit:

 - operator==/!=/[]
 - HasMember, FindMember, RemoveMember
 - StringEqual
GenericValue: add and use IsGenericValue meta function

This commit adds an IsGenericValue meta function to match arbitrary
instantiations of the GenericValue template (or derived classes).

This meta function is used in the SFINAE-checks to avoid matching
the generic APIs (operator=,==,!=; AddMember, PushBack) for instances
of the main template.  This avoids ambiguities with the GenericValue
overloads.
valuetest: always test comparisons with different allocators
GenericValue: add explicit operator!=(const Ch*)

MSVC'2005 needs an explicit overload for operator!=(const Ch*)
after the addition of the IsGenericValue SFINAE restrictions.
valuetest: test member operations with different allocators
valuetest: more testing of MemberCount
unittest.h: simplify AssertException

Some compilers warn about the missing initialisation of the std::exception
base class of the AssertException helper.  The simplest solution is to
inherit from std::logic_error instead, which provides all of the required
functionality already.
Merge pull request #127 from pah/feature/value-different-allocators

GenericValue: accept values with different allocators for read-only access