CI: add circle ci support
this adds support for using CircleCI; configured to put 4 of our builds (3.4, 3.4-slow, 2.7 compat, and 3.5 ascii), they are still on Travis ATM. They are built/tested simultaneously on CircleCI (as we get 4 containers for open-source).

CircleCI is a PITA. It has parallelism, but no build-matrix. So this took a while to get right. Don't currently use pytest-xdist as was getting some failures, but maybe can turn on at some point.

I'll come back later and remove the builds from TravisCI when everything's all connected.
cc @wesm  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15464?src=pr&el=h1) Report
> Merging [#15464](https://codecov.io/gh/pandas-dev/pandas/pull/15464?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0b4fdf988e3125f7c55aaf6e08a2dfa7d9e2e8a0?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.


```diff
@@           Coverage Diff           @@
##           master   #15464   +/-   ##
=======================================
  Coverage   90.37%   90.37%           
=======================================
  Files         135      135           
  Lines       49473    49473           
=======================================
  Hits        44709    44709           
  Misses       4764     4764
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15464?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/index.py](https://codecov.io/gh/pandas-dev/pandas/compare/0b4fdf988e3125f7c55aaf6e08a2dfa7d9e2e8a0...375667444d8805a41c04a8a7148fda5f4cbbfb71?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvaW5kZXgucHk=) | `95.32% <ø> (-0.1%)` | :x: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/0b4fdf988e3125f7c55aaf6e08a2dfa7d9e2e8a0...375667444d8805a41c04a8a7148fda5f4cbbfb71?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.36% <ø> (+0.33%)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15464?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/15464?src=pr&el=footer). Last update [0b4fdf9...3756674](https://codecov.io/gh/pandas-dev/pandas/compare/0b4fdf988e3125f7c55aaf6e08a2dfa7d9e2e8a0...375667444d8805a41c04a8a7148fda5f4cbbfb71?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). @jreback Great!