New function to recover active checks

[adrianlzt]

The new function get_active_checks() return a list of active items that
a Zabbix Agent should execute.

[coveralls]

Pull Request Test Coverage Report for Build 267

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 89.597%

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Totals
Change from base Build 262:	0.3%
Covered Lines:	267
Relevant Lines:	298

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 265

• 3 of 7 (42.86%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.09%) to 88.255%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyzabbix/sender.py	3	7	42.86%

Totals
Change from base Build 262:	-1.09%
Covered Lines:	263
Relevant Lines:	298

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💛 - Coveralls

[adubkov]

Thank you for your contribution. Could you update tests to support new functions you add?