Allow dtype='category' or dtype=None for Series with Categorical
Allows user to pass dtype='category' when creating a Series with Categorical, but produces a ValueError is any non-categorical dtype is passed.
- [x] closes #12574
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Just the one comment, +1 otherwise (assuming Travis passes). Thanks!
 ## [Current coverage](https://codecov.io/github/pydata/pandas?ref=724c3091bff822f95aedd786a44be9e85c242511) is `88.64%`

> Merging **#12636** into **master** will not affect coverage as of [`724c309`](https://codecov.io/github/pydata/pandas/commit/724c3091bff822f95aedd786a44be9e85c242511)

``` diff
@@            master   #12636   diff @@
=======================================
  Files          275      275       
  Stmts       133688   133695     +7
  Branches         0        0       
  Methods          0        0       
=======================================
+ Hit         118511   118520     +9
  Partial          0        0       
+ Missed       15177    15175     -2
```

> Review entire [Coverage Diff](https://codecov.io/github/pydata/pandas/compare/f71537ab2561ab5727008095e9685966619fa7b9...724c3091bff822f95aedd786a44be9e85c242511) as of [`724c309`](https://codecov.io/github/pydata/pandas/commit/724c3091bff822f95aedd786a44be9e85c242511)
> 
> Powered by [Codecov](https://codecov.io). Updated on successful CI builds.
 thanks!
