2.3.0 (final) New collections module + refactoring

This is the final version of 2.3.0 (as published to PyPi) - merged from `develop` branch.

Includes the following extra commits since original release:

 - (commit 600cccd) Rename test_tuple to test_collections + add TestDictObject

 - (commit fa1f4db) Re-factored dictable_namedtuple and related functions for py3.6 / 3.7 compat

**Minor updates**

 - `docs/source/conf.py` now sets `PYTHON_PATH` which helps reduce issues with Sphinx
   finding the privex package folder

 - Fleshed out `docs/source/examples.rst` with `DictObject` and `dictable_namedtuple` examples.

 - Added documentation for `privex.helpers.collections` and `tests.test_collections`

 - Added `Pipfile` and `Pipfile.lock` for use with `pipenv` during development

**Major changes**

 - Created module `privex.helpers.collections`

   - `DictObject` - A `dict` sub-class which allows keys to be read/written via
     attributes (`x.something`) as well as standard item/key notation (`x['something']`)

   - `MockDictObj` - Same as `DictObject`, but masquerades as the builtin `dict`, potentially
     allowing it to be used with certain code that expects the builtin dict type

   - `is_namedtuple` - Boolean function which returns `True` if all passed objects are named tuples

   - `dictable_namedtuple` - An improved version of the native `collections.namedtuple`, which adds additional functionality
     such as dict-like key/item access to fields, ability to cast directly to a dict, and ability to add new fields
     dynamically to an existing instance.

   - `convert_dictable_namedtuple` - Converts a `namedtuple` type class instance into a `dictable_namedtuple` instance

   - `subclass_dictable_namedtuple` - Converts a `namedtuple` type/class into a `dictable_namedtuple` type/class

 - Created unit tests for `is_namedtuple` and `dictable_namedtuple` in `tests/test_collections.py`

**BREAKING CHANGES**

 - `Mocker` has been moved from `privex.helpers.common` into `privex.helpers.collections`.
   Code which imports via `from privex.helpers.common import Mocker` will no longer work.
   Code which imports `Mocker` from the shared `privex.helpers` module (i.e. `from privex.helpers import Mocker`)
   should be unaffected.

 - `Dictable` has been moved from `privex.helpers.common` into `privex.helpers.collections`.
   Code which imports via `from privex.helpers.common import Dictable` will no longer work.
   Code which imports `Dictable` from the shared `privex.helpers` module (i.e. `from privex.helpers import Dictable`)
   should be unaffected.