Add a new method to unwraps the raw database connection

[importcjj]

Can you add a take method for mobc::Connection? @importcjj
I can give up the current connection when I found the error is it.

The code is probably like follow

impl<M: Manager> Connection<M> {
    /// Take this Connection from the pool permanently.
    pub fn take(self) -> M {}
}

And this method can help convert the redis connection to PubSub connection: https://docs.rs/redis/0.16.0/redis/aio/struct.Connection.html#method.into_pubsub

Originally posted by @biluohc in #28 (comment)

[importcjj]

I think into_inner is better than take probably, I will add it as soon as possible.