[pytest_selenium.py] Date input

* Added possibility to enter the date as tuple (Y, M, D)
* In addition to this, possibility to set the read-only date input was added
* And to support all of this, a function to get all attributes from element has been added
Singledispatch implementation for handling readonly calendar fields
Merge pull request #431 from seandst/datepicker

Singledispatch implementation for handling readonly calendar fields