Add PhoneNumber, DateOfBirth, URL variable types

[gsheni]

Add PhoneNumber, DateOfBirth, URL variable types

---

• Add PhoneNumber Variable Type
• Add DateOfBirth Variable Type
• Add URL Variable Type
• Add these types to featuretools/tests/testing_utils/mock_ds.py
• Add to docs/source/api_reference.rst
• Two PR Examples of adding new Variable Types #430 and #426

  
  class PhoneNumber(Variable):
  _dtype_repr = "phone_number"
  _default_pandas_dtype = str

class DateOfBirth(Datetime): _dtype_repr = "date_of_birth" _default_pandas_dtype = np.datetime64

class URL(Variable): _dtype_repr = "url" _default_pandas_dtype = str

[glentennis]

@gsheni , some small questions:

• how strict is the phone number format? what will we allow? do we want country code?
• same question for urls. is the url any valid url? or should there be an "http://" at the front?

[gsheni]

• We should allow any common number format.
• the url is any valid url.