issue 441: unit test for double optimize

[shadiakiki1986]

Related to #441

I followed the instructions in issue 441 (by odow) to create this PR + added 2 changes

• travis badge links updated to travis-ci.com instead of travis-ci.org (currently the badge goes to an extra redirect page)
• added a new page in the docs about testing. I think this needs some more revision, but I just wanted to get your opinion first about having such a page in the docs. It's basically the same instructions as odow laid out in the issue. I think that it'd be a good documentation for how the testing works (I personally wasn't expecting that the optimizers ran tests from the MOI package), as well as tests being tested, and where to plug in stuff in order to add a new test. Let me know if you think it's a bad idea.

I also tested that the ECOS tests would give out an error from the solve_twice if the early return in the optimize! is commented out (source).

[odow]

Nice work. To make it so we can merge some bits quicker, this could be split into three PRs.

1. The change to the README.
2. The new tests.
3. The new documentation.

Ideally, we keep PR's as small and self-contained as possible.

[shadiakiki1986]

Ok, I dropped the readme and new testing docs page from this PR

[shadiakiki1986]

Rebased branch on upstream master