Mysql adapter

Notes
- no transaction support
- uses internal pooling
The Right hand side equates to:

```
%Ecto.Adapters.Mysql.Error{
  msg: "You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''''\\'\nFROM `posts` AS p0' at line 1", 
  params: [], 
  query: "SELECT '''\\'\nFROM `posts` AS p0"
}
```

I can make the test pass by using `^"'\\"`, and the query in the error message passes when input into mysql query shell:

```
mysql> SELECT '''\\' FROM `posts` AS p0;
+----+
| '\ |
+----+
| '\ |
+----+
1 row in set (0.00 sec)
```

So I set it aside for now.