support for asio standalone

Hi @anarthal Ruben,

great work, thanks!

There's quite a big community using asio in stand-alone mode without boost. Is boost really needed or would it be possible to make a definition to support compiling without the boost framework and namespaces?

i.e.

```
#ifdef ASIO_STANDALONE
#define MYSQL_COMPLETION_TOKEN ASIO_COMPLETION_TOKEN_FOR
#else
#define MYSQL_COMPLETION_TOKEN BOOST_ASIO_COMPLETION_TOKEN_FOR
#endif

```

Thanks and best,
Roman

Update: according to grep, there's just a few includes from boost that are non-asio related, and their functionalities are covered since c++11 by std::*, except - ok - lexical cast, config and endian conversion:

```
#> egrep -Riohw "#include "|sort|uniq | grep -v 'boost/asio'
#include
#include
#include
#include
#include
#include
#include
#include
```