Add code coverage check and codecov.io badge

[meahow]

This should close #251
You might want to create separate codecov.io account for sendgrid-python project.
After this, the badge url should be changed in README.md

[SendGridDX]

All committers have signed the CLA.

[meahow]

This url need to be changed once you setup a (free) codecov.io account

[mbernier]

@thinkingserious can you please create this with the SendGridDX account?

[meahow]

The travis job is broken for py2.6

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@5abeaa6). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #352   +/-   ##
=========================================
  Coverage          ?   81.24%           
=========================================
  Files             ?        9           
  Lines             ?      981           
  Branches          ?      156           
=========================================
  Hits              ?      797           
  Misses            ?       90           
  Partials          ?       94

---

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[thinkingserious]

Hi @meahow,

We are dropping support for Python 2.6 in the next version of this SDK.

I will still be able to merge your work.

Thanks!

With Best Regards,

Elmer

[mbernier]

I attempted to merge the conflicts that popped up, hopefully it builds properly now!

[thinkingserious]

Hello @meahow,

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

[meahow]

Thanks @thinkingserious, I just did.