Shorter diff functions

shorter and more simple
Replace Datetime:: with self:: & static::
Add @throws to docblocks
removed unused var
Merge pull request #39 from semalead/diff-functions

Shorter diff functions
Merge pull request #40 from semalead/own-format-constant

Replace Datetime:: with self:: & static::
Merge pull request #41 from semalead/unused-var

removed unused var
Merge pull request #42 from semalead/docblocks

Add @throws to docblocks
Merge pull request #38 from nicholas-eden/master

Add test aid for relative terms
Fixed up readme (cleared test now mock) and added a few gotchas.