Libhxl

For the HXL 1.1 JSON formats, support reading a JSON array of objects as well as an array of arrays.

Search recursively for HXL data inside a JSON dataset.

Improve exception handling for HTTP errors.

Bug fix release before rolling out new HXL Proxy:

* fixed bug with disabling SSL checks via requests library
* improved Excel data handling — use integers instead of floats when possible, and fix bug when trying to parse numbers as dates

* added support for [JSON-flavoured HXL input](https://github.com/HXLStandard/hxl-proxy/wiki/JSON-data-format)
* added a new [fill data filter](https://github.com/HXLStandard/hxl-proxy/wiki/Fill-data-filter)
* improved input to use both MIME type and file extension to guess file type
* use character encoding from HTTP response if available
* added an optional parameter to skip SSL certification verification for remote resources with expired or self-signed certs
* added a new _is (not)_ operator for [row queries](https://github.com/HXLStandard/hxl-proxy/wiki/Row-queries) (e.g. "is number", "is not empty").
* added an optional _date format_ parameter for the [clean data filter](https://github.com/HXLStandard/hxl-proxy/wiki/Clean-data-filter)
* misc. bug fixes (see CHANGELOG)

- Fully modularised JSON specs and recipes.
- Made JSON specs and recipes work recursively.
- Fix bug that caused select filter to fail after explode filter
- Added top-level hxl.tagger() function, similar to hxl.data()
- add optional default_header arg to hxl.model.Column.parse_spec
- major overhaul of the merge_data filter: now merges _all_ columns matching the pattern supplied, and doesn't create an empty column if there are no matching columns in the merge dataset
- refactored append filter to allow multiple append files in single filter

- add timeout option for opening URLs (avoids long wait in unit tests)
- encoding fixes for Python2
- add HXLColumn.has_attribute() method
- add an optional parsed attribute to HXLRow.get() to try parsing the value according to attributes (currently supports +list)
- add experimental support for the +list attribute

Fixes to the I/O module to work properly with requests_cache.