add initial failing tests for assertArrayPart
add implementation for assartArrayPart
add stronger test for assertArrayPart, expect exceptions for invalid argument types
pass tests for assertArrayPart, throw exceptions for invalid arguments
fix docs for assertArrayPart and ArrayPart constraint
fix code style
add new tests for assertArrayPart this time with strict mode switch, cc @whatthejeff
add strict switch to assertArrayPart
rename assertArrayPart to assertArraySubset, as discussed in #1371
bump `@since` tag from 4.3.0 to 4.4.0, as defined in https://github.com/sebastianbergmann/phpunit/pull/1377#discussion_r15846100
Merge pull request #1377 from marcioAlmada/assert-array-part

Implementation for assertArraySubset #1371