switch to mime-types module to make form-data browserifyable.

The reason this module is not browserifyable is because the mime module does not use `fs.readFileSync` in a compatible way. A pull request to fix this in mime has been open for over a year, but has not merged it (nor given a good reason not to).

request itself has switched to using mime-types instead of mime, https://github.com/request/request/pull/943

The mime-types module is drop-in compatible, and I have also updated all tests and they still pass just the same.