Redesign test cases and refactor build scripts

[mliu49]

This PR contains significant updates to the test cases in RMG-tests as well as the Travis build process. No changes were made to the local testing framework.

Test cases:

• Minor renaming of tests, files now located in tests directory, with additional subdirectories available.
• New test cases were created to more comprehensively test RMG-Py features and chemistries.
• Old examples were retained and relocated to the tests/examples directory

Travis build:

• Now using build stages, which allows us to run test cases across multiple (parallel) jobs. Importantly, RMG compilation can be done once and cached to avoid excess overhead in subsequent jobs.
• Travis can now automatically update the status of commits to indicate whether or not they pass RMG-tests. Note: passing only indicates that all the jobs completed, but does not account for any differences which may exist between the benchmark and testing branches.

Reviewing:

• Skim through the code
• Look through the new example files for any mistakes
• Take a look at the status report at ReactionMechanismGenerator/RMG-Py#1228
• Take a look at an example Travis build, link available in the status section of 1228

[alongd]

I went over the tests, everything looks good! (did not go over the shell files)

What's our plan for the long tests? The long nitrogen test crashes when it calls SA, I'll open an issue

[mliu49]

I think I'll remove the long nitrogen test for now, especially if it's crashing. I'll put it on a different branch and we can continue working on it.