Revert "Make testing a separate step during build"

This reverts commit 83be2ae6d70851cdea31802a0a886adcff458aeb.
Test against one OTP release
Make a separate make rule for building with rebar
Add ArgumentError exception
Fix from() implementation
Move some code around a bit
Implement proper ISO date formatting
Implement proper RFC date formatting
Test is_valid()
Implement normalize(_, :clamp)
Leave only one flavor of normalize()
Test replace()
Fix shift() signatures