PR for v.1.3.0

[hcpchris]

• ENHANCEMENT: Modified the way that data points are serialized to JavaScript literal objects. Now, they are serialized to a JavaScript array if their configured properties are those that Highcharts (JS) supports in JavaScript array notation. Otherwise, the code falls back to serialize the data point as a JavaScript object literal. This change is intended to improve performance and reduce the size of the serialized data. (Closes Serialize DataCore descendents to array of primitives rather than object when possible #32 )
• ENHANCEMENT: Added __repr__() method for Highcharts Core for Python classes.
• ENHANCEMENT: Added __str__() method with special handling for difficult-to-read classes.
• ENHANCEMENT: Added Chart.get_script_tags() to retrieve Javascript <script> tags.
• ENHANCEMENT: Added utility_functions.to_snake_case() function.
• BUGFIX: Fixed incorrect serialization of datetime and Pandas Timestamp objects in .to_dict() and .to_json().
• BUGFIX: Fixed incorrect serialization of EnforcedNull in .to_dict() and .to_json().