Fix #313 Assertion In `Pow10.h` is triggered in Document::Parse

Fixed to build on older versions of 32-bit MSVC

RapidJSON v1.0.0

Introduce option to select default gtest installation

This will introduce RAPIDJSON_BUILD_THIRDPARTY_GTEST option. If it is set to
TRUE, cmake will look for GTest installation in `thirdparty/gtest` before
looking in other places.

Current default value (OFF) for RAPIDJSON_BUILD_THIRDPARTY_GTEST represents
previous behaviour when system-wide gtest installation is used whenever
possible.

This commit will as well eliminate problem described in #309 when source
directory found is `thirdparty/gtest` while include files are found
system-wide. This however won't give the user possibility to select gtest
installation to use.

Avoid calling memcpy with NULL pointers

According to the C/C++ standards, calling `memcpy(NULL, NULL, 0)` is
undefined behaviour. Recent GCC versions may rely on this by optimizing
NULL pointer checks more aggressively, see [1].

This patch tries to avoid calling std::memcpy with zero elements.
As a side effect, explicitly return NULL when requesting an empty block
from MemoryPoolAllocator::Malloc.

This may be related to #301.

[1] https://gcc.gnu.org/gcc-4.9/porting_to.html

Add build status

Add coverall badges

100% line of code coverage

Improve code coverage for iterative parsing

Assert on impossible state transition in Transit(); Put the last case and all non-enumerated cases(also supply assertion for them) in  for code coverage.