credentials must be set using travis's per repository settings.
currently only chrome and firefox are tested, we should add tesing on ms edge and IE.
Follows on from work in #1105
Move karma config files to browser-tests and split out some
of the config file to make adding browserstack easier.
`npm run test:browser` is more descriptive than `npm run karma`, does not
tie mathjs into using karma and will allow a new npm script to run tests
on browserstack also using karma.
