Add new MERGE import mode

[colinodell]

Implements/resolves #31

Also includes a fix for this bug:

$value1 = ['test' => 'string'];
$value2 = ['test' => ['array']];
$value = Util::mergeAssocArray($value1, $value2);

// Before fix: ['test' => 'atring']
// After fix:  ['test' => ['array']]