Spidermon

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- Allowed to add absolute location for custom templates

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- Added new StatsCollector that access stats data from previous spider executions.
- Added new setting to define the max number of unwanted HTTP status codes allowed in built-in monitor.
- Improved validation error messages with JSON Schema when additional fields are found.
- Made possible to retrieve JSON schema files from external locations.
- Included documentation of periodic monitor suites.
- Fixed bug caused by new slackclient release.
- Other small documentation improvements.

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- Add set of built-in basic monitors with the most common test methods to allow
start monitoring spiders more straightforward.
- Add SendSentryMessage action to send notifications to Sentry containing the
results of Spidermon execution.
- Add SPIDERMON_ENGINE_STOP_MONITORS setting to list monitors to be executed
when the Scrapy engine is stopped.
- Fix bug that prevented the use of custom model-level validators in schematics models.
- Refactor JSONSchemaValidator to allow select different versions of JSON Schema.
- Refactor requirements in setup.py to include missing required dependencies.
- Fix bug caused by backward incompatible change in jsonschema 3.0.0.
- Fix example code of tutorial.
- Install documentation improvements.

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- Remove CreateJobReport action.
- Include new documentation and tutorial code.
- Rename internal method in MonitorRunner to fix typo.

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- Support universal wheels.
- Skip authentication and recipient settings when running in fake mode.

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- Add SPIDERMON_EMAIL_CONTEXT setting to pass custom contexts to email actions.
- Add support for Schematics 2.1.0.