Add `valuelike` custom lookup

[DavidMuller]

Build Failed Update

Looks like my build failed on versions of Django != 1.7, because custom lookups were a feature added for 1.7.

Happy to rework this to get it going on all versions--let me know what you guys think!

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Overview

This PR adds a new lookup: valuelike

Motivation:

Currently, the contains lookup, only retrieves database rows that have key/value pairs that exactly match your inputted dictionary. For example, running a __contains={'my_key': 'my_value'} only returns database rows that have an exact match on the key/value pair 'my_key': 'my_value'

Changes:

The valuelike lookup, e.g. __valuelike={'my_key': 'my_value'}, will retrieve a QuerySet containing rows where 'my_key' has a value LIKE %my_value%.

• If 'my_key' is not in any of the database rows, returned QuerySet will be empty
• If 'my_key' is in one or more database rows, checks value of 'my_key' for a match with LIKE %my_value%.

  Usage Example:

MyModel.objects.filter(hstore_attribute__valuelike={'my_key': 'string I am looking for'})

Case Sensitivity

This PR also adds the ivaluelike lookup which is identical to valuelike except that it uses the case insensitive version of the LIKE clause (ILIKE).

[coveralls]

Coverage decreased (-8.5%) when pulling 685825fd71bfa191970ed13e03aa209f9f61b2d1 on DavidMuller:master into 7ccfe86696f43b6b8d42b70126ba553302c9a1dc on djangonauts:master.

[DavidMuller]

Ah--looks like my build failed on versions of Django != 1.7, because custom lookups were a feature added for 1.7.

Happy to rework this to get it going on all versions--let me know what you guys think!

[nemesifier]

Did you get anywhere with this feature? Are you using it in production?

[DavidMuller]

@nemesisdesign -- played around with it for a while, but ultimately decided to leverage Django 1.8's built-in hstore support