Allow getting params the same way data() works 

I could improve this to do the `hash::insert()` to replace `addParams()` as is done in `data()`
make the default return false so it matches previous use, improve tests for new method
adding support to write values to param like can be done with data(), method returns $this as does ->data() when writing
adding doc block for tests related to CakeRequest::param()
Merge pull request #3548 from dogmatic69/patch-3

Allow getting params the same way data() works