Add/Update Badges on README

[mptap]

• - Put the logo at the top of the list
• - Added License
• - Added Twitter, Github, and License badges

@mbernier edit:
closes #293

[SendGridDX]

All committers have signed the CLA.

[mbernier]

Looks good to me, thank you!

[jitendra3109]

@mptap Check your PR for Travis-ci failing.

[mptap]

@jsroyal: That's a known problem caused by Gradle builds in Travis.
@mbernier: ^^ Let me know if there's anything I can do.

[mbernier]

@mptap if this is something you can fix, we would love you too - it is outside my expertise and @thinkingserious is SLAMMED right now (we have had almost 400 PRs in 21 days)

Double check the PRs though, just in case someone else has picked it up.

[thinkingserious]

Hello @mptap,

Thanks again for the PR!

It's HACKTOBERFEST! We want to show our appreciation by sending you some special Hacktoberfest swag. If you have not already, could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX