FileFields now have different browser/server behaviour. Closes #27

FileFields now validate that a file is selected when required=true in browsers

- Added an env module which uses the presence of process to set the environment
  - Updated browserify config in gulp file to not pull in a process shim due to the process reference
- In browsers, the FileInput widget now returns its value from data not files
- In browsers, FileField now only validates the presence of a selected file
- Split tests affected by these changes into browser and server modules and added setup/teardown to override & restore the environment flag required by each test module