Invenio-query-parser

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- Removes check for Flask application context from
`build_valid_keywords_grammar` function in favor of new `keywords`
argument.
- Elastic search DSL walker returns instance of `elasticsearch_dsl.Q`
instead of `dict`.

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- Allows match unit to iterate over Sequences, i.e. tuple.
- Removes distinctions between double and single quotes.
- Adds support for allowed keywords.
- Removes nestable keywords (to be re-added in the future).
- Removes optional space after a keyword.
- `AND`, `OR` and `NOT` are only considered as keywords when
written in caps.

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- Default key getter returns default value instead of raising key
error if the key is not found inside the dictionary.

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- Removes support for Python 2.6.
- Adds **experimental** support for Elasticsearch query generation.
- Adds `MatchUnit` walker for testing data against a query.
- Adds Python 3.5 classifier and enables it in test matrix.
- Fixes 'not' operator usage at the beginning of the query and after
an 'or' operator.

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- Allows search keywords to contain dots to point to subfield content,
i.e. `author.name: Ellis`.

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- Initial public release.
- Adds Python2/Python3 compatibility layer. (2)
- Adds new Sphinx documentation page. (3)
- Adds simple inheritance for visitor pattern and separates SPIRES syntax
parser to contrib module. (7)