Implementation of Kruskal's Minimimum Spanning Tree construction algorithm

[goldragoon]

Since that, in sparse graph kruskal's algorithm works better in general.
But this beloved package dont't have krulskal's algorithm implementation.
So I add kruskal's algorithm using disjoint set datastructure class(UnionFind) to find minimum edge without cycle.

If any suggestion to improve my code, please note it.

[coveralls]

Pull Request Test Coverage Report for Build 21

• 50 of 52 (96.15%) changed or added relevant lines in 3 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 93.732%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/pygraph/algorithms/minmax.py	28	29	96.55%
core/pygraph/classes/unionfind.py	21	22	95.45%

Files with Coverage Reduction	New Missed Lines	%
core/pygraph/algorithms/minmax.py	3	95.76%

Totals
Change from base Build 17:	-0.2%
Covered Lines:	1271
Relevant Lines:	1356

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💛 - Coveralls

[agroszer]

thank you,

but please fix travis py 3.x tests and add tests for your added code to keep coverage at least level

[goldragoon]

@coveralls
Thanks for replying.
I will fix it asap.

[agroszer]

thank you