Update RAPIDJSON_ALIGN() to always align on an 8-byte boundary
unless otherwise overridden.

On some platforms (such as ARM), 64-bit items (such as doubles and
64-bit integers) must be aligned to an 8 byte address, even though the
architecture is only 32-bits. On these platforms, MemoryPoolAllocator
must match the malloc() behavior and return a 8 byte aligned allocation.
This eliminates any alignment issues that may occur at the expense of
additional memory overhead.

Failure to do so caused a SIGBUS signal when calling
GenericValue::SetNull(). The size of the data_ member of the
GenericValue class is 16 bytes in 32-bit mode and its constructor
requires an 8-byte aligned access.

While parsing a JSON formatted string using Document::ParseStream(), a
stack object containing GenericValue items was constructed. Since the
stack was 8-byte aligned, the constructor calls would succeed. When the
lifetime of the object ends, SetObjectRaw() is invoked. This triggered
an allocation with 4-byte alignment to which the previously 8-byte
aligned GenericValue array was copied. After this, any call to a
GenericValue API that triggered the constructor and thus the placement
new operation on the Data type member would trigger a SIGBUS.
Signed-off-by: Veselin Georgiev <veselin.georgiev@garmin.com>
Signed-off-by: Joshua Watt <Joshua.Watt@garmin.com>

Added unit tests for alignment macros.
Fixes #418

git-svn-id: https://rapidjson.googlecode.com/svn/trunk@2 c5894555-1306-4e8d-425f-1f6f381ee07c