Add datacontainer based AccountStore and make it the default

[csteeg]

Add datacontainer based AccountStore and make it the default. Added o…ption to user AccountStore.CreateFileBased() if an app has existing accounts stored in files

To be completely backwards compatible, those should be swithed around ofcourse

Xamarin.Auth Pull Request

Fixes #327 .

Checklist

• [X] I have included examples or tests => existing samples work transparently with this store
• [ ] I have updated the change log
• [ ] I am listed in the CONTRIBUTORS file
• [X] I have cleaned up the commit history (use rebase and squash)

Changes proposed in this pull request:

• Use a datacontainer as default store on UWP

[moljac]

Thanks a lot for your effort in submitting this effort, but can you provide minimal sample, so we can test this PR.

Namely due to our limited resources it is highly unlikey that we'll be able to do it ourselves soon.

[csteeg]

@moljac This pull-request will switch the default accountstore on UWP to the container based version instead of the file-based version, so the existing samples should still be working then. Do you mean a different example?