Add startOfYear() and endOfYear() functions
Add start/end of year/decace/century functions

Close and replace PR https://github.com/briannesbitt/Carbon/pull/63
Also update readme
Add average function
Merge pull request #65 from lucasmichot/feature/average

Add average function
Merge pull request #64 from lucasmichot/feature/start-end

Add new start / end functions
Might as well use the const rather than a magic number!
Moved average to the modifiers and documented. Fixed human diff default when null.
Merge branch 'master' of https://github.com/lucasmichot/Carbon