CLN/DOC: Refactor timeseries.rst intro and overview
Refactoring `timeseries.rst` introduction and overview to give a better introduction into pandas timeseries functionality:

- Introduce a wider range of examples
- Overview all scalar and array classes, related data types, and range methods, and `NaT` in the beginning

 

<img width="777" alt="screen shot 2018-09-15 at 10 56 12 pm" src="https://user-images.githubusercontent.com/10647082/45593464-12d62300-b93c-11e8-9d13-71d3cb374ac3.png">
<img width="773" alt="screen shot 2018-09-15 at 10 56 23 pm" src="https://user-images.githubusercontent.com/10647082/45593467-18cc0400-b93c-11e8-87e6-6bd2f7dead96.png">
<img width="774" alt="screen shot 2018-09-15 at 10 56 32 pm" src="https://user-images.githubusercontent.com/10647082/45593468-1cf82180-b93c-11e8-8428-eecebade3769.png">
<img width="776" alt="screen shot 2018-09-15 at 10 56 40 pm" src="https://user-images.githubusercontent.com/10647082/45593470-208ba880-b93c-11e8-91a5-1aa4b59fdfd7.png">
 Good suggestions @datapythonista. Incorporated all your comments.  Thanks @datapythonista @WillAyd. 