Prepare to release the new version 1.4

[peter279k]

As title, we prepare to release the new version 1.4.
However, something need to be reviewed before releasing the new version.

To @joefearnley, I've almost done the testings and I modify the README.md now.
Can you review the sample code and help me adding the SendGrid sample code?
BTW, I also use the Mock to test the all HTTP API request methods.
You can see more details about this in the tests folder and the changed log section in README.md

Thanks.

[joefearnley]

Hi @peter279k, yes, I will review the changes and add the SendGrid example.

Also, I have have written the code for Mailjet integration. I will submit a PR later on today.

[joefearnley]

Hi @peter279k,

I submitted a PR (#9). Let me know if I missed anything.

Thanks!