Add VALIJSON_USE_EXCEPTIONS mode

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

tests/test_validation_errors.cpp

@@ -36,7 +36,11 @@ TEST_F(TestValidationErrors, AllOfConstraintFailure)
     // Parse schema document
     Schema schema;
     SchemaParser schemaParser;
-    ASSERT_NO_THROW( schemaParser.populateSchema(schemaAdapter, schema) );
+#if VALIJSON_USE_EXCEPTIONS
+    ASSERT_NO_THROW(schemaParser.populateSchema(schemaAdapter, schema));
+#else
+    schemaParser.populateSchema(schemaAdapter, schema);
+#endif
 
     // Load test document
     rapidjson::Document testDocument;