This patch adds support for a new Cmake option,
VALIJSON_USE_EXCEPTIONS. If specified and set to `0`, Valijson will
disable all exception throwing, add the `-fno-exceptions` compiler flag,
and print to std::err and abort where exceptions would have been thrown.
NOTE: to set the value of a CMake option, the easiest way is to modify
the appropriate source line in the <build folder>/CMakeCache.txt file.
Bug: #106
When resolving a json pointer and use an empty string as a
referenceToken, the value is coerced as an empty object and a temporary empty
object is returned from 'asObject()'. Then we used to compare 2
iterators from 2 differents temporary empty objects.
