Update prepare and transactions methods to use context

[avinassh]

I have added PrepareContext and BeginTx methods. I have also replaced context.TODO with context.Background

Following methods are required because of interface, but they are not used:

• Begin
• Prepare
• In stmt struct Exec and Query

I have confirmed this by keeping panic() calls and running the test suite.

Also, I updated the example snippet to use ctx in every call and included a sample of prepared statements

[avinassh]

It's better if we merge #24 first, then we can run the tests in this PR to make sure everything is alright

[haaawk]

Thanks @avinassh. Looks mostly good but I think the PR needs some cleanup. Please remove commits that introduce changes that are removed later. Also please remove your credentials :)

[avinassh]

@haaawk hey, I have cleaned up and squashed all the commits

[haaawk]

Thanks @avinassh. Looks much better. Just one last question.