Travis build

[jesperronn]

I added a .travis.yml file and hooked up my fork to build on every commit.

• travis-ci builds of Stackedit
• skip running tests on Travis until Stackedit has a test suite
• @benweet should activate Travis Github hook for Stackedit project

[jesperronn]

Note that you can actually see build status for this branch in my fork:

https://github.com/jesperronn/stackedit/tree/travis_ci

and https://travis-ci.org/jesperronn/stackedit/builds/53183403

And here is the status badge to follow the status:

[jesperronn]

Updated this with new Travis (that should run faster).

See https://travis-ci.org/jesperronn/stackedit/builds/70342949 for results on this build.

Note also that I decided to keep here the updated npm convenience commands: npm start and npm postinstall, since they are really, really useful. (simplifies install also for Travis)
https://github.com/benweet/stackedit/pull/729/files#diff-b9cfc7f2cdf78a7f4b91a753d10865a2R39

The other thing is developer-guide.md where I removed trailing whitespace and updated the guide on the section that simplifies setup (as stated above).

If you still don't want to merge with this simplification, then let me know and I'll split this PR for simplification. @benweet, please give me that feedback, thanks 👍

[RoyiAvital]

Any chance to create a local application from this?
Namely something which runs on the local computer and edits local files?