Add Table::addAssociations()

It allows setting multiple associations using a single function call
using an array argument and facilitates DRYer app code.
Docbloc fix.
Improve docblock example.
Removed unneeded null argument check.
Update doc blocks for addAssociations()
Fix method name in docblock
Merge pull request #5314 from cakephp/3.0-associations

3.0 - Add Table::addAssociations()