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.

Use assertion for impossible case(The Predict() can ensure the token is ColonToken, otherwise it would be marked as Error state. So there is no need to check ColonToken again).