Add guerilla to codecov.io.

[seyeojumu]

Please add the guerilla repo to coveralls:

• enables Add coverage reporting, using coveralls, to guerilla #46
• add the markdown badge to Readme.md

[nickwph]

just wondering should we use Codecov.io too
they have a nice (official) chrome extension to visualize coverage on github.
https://codecov.io/#features

[seyeojumu]

that does look pretty cool. @nickwph would you rather use that?

[seyeojumu]

add to codecov, instead of coveralls

[nickwph]

@seyeojumu sorry for the delay.
personally i like codecov because of the chrome extension and it's a bit cheaper for private repo.
but i am okay with any of these.