Add Base64 support for FileStorage

1. Add Base64 support for reading and writing XML\YML file.

The two new functions for writing:

```cpp
void cvWriteRawData_Base64(cv::FileStorage & fs, const void* _data, int
len, const char* dt);
void cvWriteMat_Base64(cv::FileStorage & fs, cv::String const & name,
cv::Mat const & mat);
```

2. Change YML file header form `YAML:1.0` to `YAML 1.0`. (standard
format)

3. Add test for Base64 part.

modules/core/test/test_io.cpp

@@ -574,7 +574,7 @@ TEST(Core_InputOutput, FileStorageKey)
     EXPECT_NO_THROW(f << "key1" << "value1");
     EXPECT_NO_THROW(f << "_key2" << "value2");
     EXPECT_NO_THROW(f << "key_3" << "value3");
-    const std::string expected = "%YAML:1.0\nkey1: value1\n_key2: value2\nkey_3: value3\n";
+    const std::string expected = "%YAML 1.0\n---\nkey1: value1\n_key2: value2\nkey_3: value3\n";
     ASSERT_STREQ(f.releaseAndGetString().c_str(), expected.c_str());
 }