Use closure actions

[kstinson14]

As of ember 3.4, the use of sendAction is deprecated in favor of closure actions. I updated the usage here accordingly, along with the README and tests. I opted to include no failsafe if a user doesn't pass the required actions to the file-picker component. If they set multiple to true and don't pass a filesLoaded action, an error will be thrown when handleFiles is called. Likewise, if multiple is not set and they don't pass a fileLoaded action, an error will be thrown. This could easily be changed if desired.

[fsmanuel]

@kstinson14 Thanks a lot! I'll have a look why the tests are failing.