Document the datetime colum datatype.

Datetime column types return/accept Carbon/DateTime instances.
Don't document PEAR installer.

PHPUnit is removing the PEAR installer support this year, pretend it
doesn't exist.
Additional updates to testing docs.
Document using phpunit.xml to define test suites.
Update Jenkins setup for 3.0
Merge pull request #1309 from cakephp/3.0-carbon-docs

3.0 carbon docs