Reorganized some features. Try to make the text clearer in features and
tutorial.

 * Support for both in-source and out-of-source builds
 * Set library version to 0.12 to map Debian package
 * Add separate options to build tests, examples and documentation
 * Add pkgconfig lookup support (if installed with `make install`)
 * Add CMake lookup support (if isntalled with `make install`)
 * Add Google Test Source lookup
 * Add CTest support for running tests (use `make test` or `ctest -V`)

In the original disambiguation fix for `GenericValue::operator[]` (#170),
the documentation has been missing, which led to quite badly rendered
Doxygen pages.

During a cleanup, I've realized that a much simpler disambiguation is
possible:

````cpp
  GenericValue& operator[](SizeType idx); // array
  template <typename T>
  GenericValue& operator[](T* name);      // object
````

This approach works, as non-template functions are preferred over
template functions.

In order to improve the error messages, the pointer type is restricted
to `(const) Ch`.

Update `tutorial.md` to drop the ambiguity warning.

Closes: https://github.com/miloyip/rapidjson/issues/142

Correct `PushBack()` example