Special handling AssignmentExpression with a MemberExpression on the right node (fixes #124)

[rpl]

In this pull requests there are:

• a couple of additional test cases to explicitly cover the issue describe in #124
• a draft for a proposed approach to deal with this scenario (to be evaluated and discussed as part of this pull request):
  • while validating a CallExpression, if we do have an AssignmentExpression as the callee node
  • check if the AssignmentExpression has a MemberExpression on the right and, if it does, validate a fake call expression that does have the memberExpression as the callee node.

[rpl]

@mozfreddyb I just updated this pull request based on the review comments and also rebased it on top of the current master to resolve the merging conflicts due to the changes in the test file. This should now be ready to go.