Datadog

* [BUGFIX] DogStatsD: Do not send empty UDP packets, [264][] (thanks [Tenzer][])
* [FEATURE] API: Add support for distributions, [312][]
* [FEATURE] ThreadStats: Add support for distributions, [312][]
* [OTHER] Remove `simplejson` 3p dependency, [304][], [309][] (thanks [alexpjohnson][])

* [BUGFIX] Dogshell: Submit `--date_happened` timestamp when posting events, [287][], [301][] (thanks [gplasky][])
* [FEATURE] API: Add [search](https://docs.datadoghq.com/api/?lang=python#monitors-search) and [groups search](https://docs.datadoghq.com/api/?lang=python#monitors-group-search) methods to the `Monitor` resource, [299][]
* [IMPROVEMENT] Dogshell: Set API and APP keys with environment variables, [228][] (thanks [taraslayshchuk][])
* [IMPROVEMENT] DogStatsD: Prevent an issue that was causing the `timed` context manager object from overwriting a method with an instance variable, [263][] (thanks [florean][])
* [OTHER] Include tests in PyPI tarball, [259][] (thanks [dotlambda][])

**New API endpoint: https://api.datadoghq.com/api**

The Datadog API client now uses https://api.datadoghq.com/api endpoint instead of https://app.datadoghq.com/api.
See [257][] for more details.

* [BUGFIX] API: Close requests' sessions to limit memory usage, [272][] (thanks [thehesiod][])
* [BUGFIX] Dogwrap: Fix `TypeError` exceptions when truncating `stdout`, `stderr` with Python 3, [260][], [267][] (thanks [cabouffard][], [glasnt][])
* [FEATURE] DogStatsD: Add client level tags to status checks, [279][] (thanks [marshallbrekka][])
* [FEATURE] DogStatsD: Add support for `statsd_socket_path` option in `initialize` function, [282][]
* [IMPROVEMENT] Dogwrap: Default output encoding to UTF-8, [268][] (thanks [glasnt][])

**Search hosts: `Infrastructure.search` is deprecated**
The `Infrastructure.search` method is deprecated in favor of the new `Hosts.search` method.

* [BUGFIX] API: Prevent exception contexts from logging URLs and credentials, [266][]
* [FEATURE] API: Add `search` and `totals` methods to the `Hosts` resource, [261][]

* [FEATURE] API: New `DashboardList` resource, [252][]

**ThreadStats: metric type change**

`ThreadStats` count metrics (produced from the `increment`/`decrement` and `histogram` methods) are now reported with the `count`/`rate` metric type, instead of `gauge`.
As a result, for the corresponding metrics:
1. Metric queries can use the `.as_count()`/ `.as_rate()` functions to switch between count and rate representations.
2. The default time aggregation uses a sum instead of an average. **This may affect the representation of existing metric queries, thus, monitors' definitions and metric graphs.**

See [242][] (thanks [nilabhsagar][]) for more details.


* [BUGFIX] ThreadStats: Send count metrics with `Rate` metric type, [242][] (thanks [nilabhsagar][])
* [IMPROVEMENT] ThreadStats: Flush all metrics on exit, [221][]