Added getMonogId()

Note, I have to move primaryKey() function from EMongoDocument to EMongoModel
to pass all tests.

Without changing primaryKey I had this errors:
````
There were 4 errors:

1) MongoDocumentTest::testOneRelation
CException: User and its behaviors do not have a method or closure named "primaryKey".

/home/pahanini/Projects/yii/framework/base/CComponent.php:265
/home/pahanini/Projects/MongoYii/EMongoModel.php:102
/home/pahanini/Projects/MongoYii/EMongoDocument.php:104
/home/pahanini/Projects/MongoYii/EMongoModel.php:324
/home/pahanini/Projects/MongoYii/EMongoModel.php:324
/home/pahanini/Projects/MongoYii/EMongoModel.php:32
/home/pahanini/Projects/MongoYii/tests/MongoDocumentTest.php:198

2) MongoDocumentTest::testManyRelation
CException: User and its behaviors do not have a method or closure named "primaryKey".

/home/pahanini/Projects/yii/framework/base/CComponent.php:265
/home/pahanini/Projects/MongoYii/EMongoModel.php:102
/home/pahanini/Projects/MongoYii/EMongoDocument.php:104
/home/pahanini/Projects/MongoYii/EMongoModel.php:324
/home/pahanini/Projects/MongoYii/EMongoModel.php:324
/home/pahanini/Projects/MongoYii/EMongoModel.php:32
/home/pahanini/Projects/MongoYii/tests/MongoDocumentTest.php:204

3) MongoDocumentTest::testWhereRelation
CException: User and its behaviors do not have a method or closure named "primaryKey".

/home/pahanini/Projects/yii/framework/base/CComponent.php:265
/home/pahanini/Projects/MongoYii/EMongoModel.php:102
/home/pahanini/Projects/MongoYii/EMongoDocument.php:104
/home/pahanini/Projects/MongoYii/EMongoModel.php:324
/home/pahanini/Projects/MongoYii/EMongoModel.php:324
/home/pahanini/Projects/MongoYii/EMongoModel.php:32
/home/pahanini/Projects/MongoYii/tests/MongoDocumentTest.php:218

4) MongoDocumentTest::testFunctionalRelation
CException: User and its behaviors do not have a method or closure named "primaryKey".

/home/pahanini/Projects/yii/framework/base/CComponent.php:265
/home/pahanini/Projects/MongoYii/EMongoModel.php:102
/home/pahanini/Projects/MongoYii/EMongoDocument.php:104
/home/pahanini/Projects/MongoYii/EMongoModel.php:324
/home/pahanini/Projects/MongoYii/EMongoModel.php:324
/home/pahanini/Projects/MongoYii/EMongoDocument.php:96
/home/pahanini/Projects/MongoYii/tests/MongoDocumentTest.php:225
/home/pahanini/Projects/MongoYii/tests/MongoDocumentTest.php:225

FAILURES!
Tests: 44, Assertions: 407, Errors: 4.
````