Add basic Travis CI configuration

I added a basic Travis CI configuration to make the build status more
transparent. I added a small logo to the bottom of the README with the
status and a link to the build.

@evancz after merging this you'd have to go to https://travis-ci.org
and enable the build for this project, just takes a couple of clicks.
See http://about.travis-ci.org/docs/user/getting-started/.

I tested this on my own github account and the configuration worked
(ie, it showed the same build errors on Travis that I'm experiencing
locally).
Add the always function

I want it, I do not want to rename constant, and I think it is a good
name.
resolve conflict in README!
Merge branch 'jsl-add_travis' into dev
Get rid of the word "simply". It is a condescending word.
Move build status higher up
Add a test for port parsing and types

Partly addressing #442
Get rid of experimental code for handling "deriving"
corrected documentation of keepIf and friends
change Mouse.isClicked to adhere to its documentation
Use Build.Print to show errors
cosmetic: get rid of a (>>) in favor of a do-block
Rename the dummy paths file so that it does not register as C code!
Add check that all ports appear in the "main" module

It is unclear what it means if ports are used in sub-modules, so to be
cautious, we want to rule it out in the first release. As people use
the API it’ll become clear how sub-module ports are necessary and what
problems they might cause.
Attempt to address #463

@ jvoigtlaender, does this clarify things enough?
Try to address #464
Merge pull request #462 from jvoigtlaender/Mouse.isClicked-correction

change Mouse.isClicked to adhere to its documentation
Revert "Merge pull request #462 from jvoigtlaender/Mouse.isClicked-correction"

This reverts commit 26b7fc617f68301ad936929e3db410438b13954e, reversing
changes made to 9be0abd01889256a337e829d9c938f9df28633ae.
Merge pull request #461 from jvoigtlaender/keepIf-docu

corrected documentation of keepIf and friends
Correctly update model, accounting for possibly dropped frames

Thanks to @johnpmayer for discovering this issue!

Since draw is async, it was possible to update the model without
updating the underlying DOM stuff. This could cause issues on its own,
but if you need to pass info from model to model, that is also lost
when this happens.