Weakly capture self to avoid leaks (#36)

Extend date decoding strategy for possible fractional seconds (#35)
* Add support for fractional seconds in dates
If the WriteFreely instance returns dates with fractional seconds (e.g., 2022-09-28T11:55:39.886581893Z), the date-decoding strategy fails and an error is thrown.
* Switch formatOptions depending on fractional seconds
* Bump minimum OS versions

Fire completion block failures instead of returning silently (#33)
* Remove unnecessary print statements
* Fire completion block failures instead of returning silently
* Add missing return statement
* Add explicit early returns

Update sample code

Merge pull request #32 from writefreely/add-arena-sample
Add sample code for Swift Package Index playground

Merge pull request #29 from writefreely/fix-build-errors
Fix build errors and add back testing files

Refactor WFClient for better maintainability and testability

Merge pull request #24 from writeas/extend-move-api-to-drafts
Extend move API to drafts

Merge pull request #22 from writeas/return-error-on-login-404
Don't return early if we get a 404 on login attempt

Merge pull request #19 from writeas/add-local-WFUser-initializer
Adds a simple memberwise-ish initializer for stored token/username