Enhance model.validate with viewName

[Daniel Zhitomirsky]

Intro
Now model.validate takes the only parameter 'instance':

model: {
...
 validate: instance => {

 }
}

In certain cases it is needed to know from which validation is called (e.g. validating PK unique key).

ToDo
So, please extend validation function with the next api:

model: {
 validate: ({instance, viewName}) => {

 }
}

[Andrei Mourzenkov]

Relevant Mocha tests still need correction. They are describe.skip-ed now.

[Egor Stambakio]

This was merged into #230 with fixed tests.

[Andrei Mourzenkov]

Change the function arguments to

model: {
  validate: ({persistentInstance, formInstance, viewName}) => {
    ...
  }
}