DOC: Update DateOffset intro in timeseries.rst
Updated `timeseries.rst` introduction to DateOffsets

* Explain relationship with `Timedelta`
* Clarify explanation of rollforward/back
* Bring all the offsets and their aliases to the front

Misc fixes:

* Period dtypes are preserved in Series & DataFrames
* Rendering for timezone nonexistent examples
<img width="792" alt="screen shot 2018-10-27 at 3 52 43 pm" src="https://user-images.githubusercontent.com/10647082/47610050-7c982100-da00-11e8-9178-f8e34d47e093.png">
<img width="805" alt="screen shot 2018-10-27 at 3 52 57 pm" src="https://user-images.githubusercontent.com/10647082/47610051-815cd500-da00-11e8-8bd9-4ca63414b267.png">
<img width="800" alt="screen shot 2018-10-27 at 3 53 10 pm" src="https://user-images.githubusercontent.com/10647082/47610052-83bf2f00-da00-11e8-9704-9fe5828f7adf.png">
 @datapythonista if you could have a look # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23385?src=pr&el=h1) Report
> Merging [#23385](https://codecov.io/gh/pandas-dev/pandas/pull/23385?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fe52d9f29416dd7d19c04ba7d47434b68c411bbd?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23385/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23385?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23385   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         161      161           
  Lines       51318    51318           
=======================================
  Hits        47339    47339           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <ø> (ø)` | :arrow_up: |
| #single | `42.31% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23385?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/23385/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.37% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23385?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23385?src=pr&el=footer). Last update [fe52d9f...1f159b1](https://codecov.io/gh/pandas-dev/pandas/pull/23385?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Updated based on the review.

Also change the page's `from datetime import datetime, timedelta, time` to `import datetime` and offsets are called like `pd.offsets.*` instead of them getting start imported from the top. can you post a rendering of this? <img width="806" alt="screen shot 2018-10-30 at 9 57 07 am" src="https://user-images.githubusercontent.com/10647082/47735693-4807ad80-dc2a-11e8-9b2c-bc4256483083.png">
<img width="801" alt="screen shot 2018-10-30 at 9 57 14 am" src="https://user-images.githubusercontent.com/10647082/47735699-4a6a0780-dc2a-11e8-9251-e740797ba839.png">
<img width="802" alt="screen shot 2018-10-30 at 9 57 24 am" src="https://user-images.githubusercontent.com/10647082/47735713-4f2ebb80-dc2a-11e8-9d3b-a34b5a406995.png">
<img width="796" alt="screen shot 2018-10-30 at 9 57 34 am" src="https://user-images.githubusercontent.com/10647082/47735719-51911580-dc2a-11e8-93cf-2d4994cc3700.png">
<img width="799" alt="screen shot 2018-10-30 at 9 57 40 am" src="https://user-images.githubusercontent.com/10647082/47735724-535ad900-dc2a-11e8-9f55-a2c5cf909177.png">
 lgtm. over to you @jorisvandenbossche  @jorisvandenbossche any additional comments? @jorisvandenbossche  @jorisvandenbossche do you mind checking if your comments have been addressed and this can be merged? We're making other changes to `timeseries.rst` and would be nice to avoid conflicts as much as possible. I'll push this in tomorrow if there are no other comments. I can always follow up after merging this PR. thanks @mroeschke 