Member groups, classes, members population for search results

#### Purpose

Implement population of members for `SearchCategories` collection. Also added fake implementation of classes population for test purposes.

#### Modifications

Implemented functions `AddMemberToGroup` and `AddClassToGroup` of `SearchCategory` class.

Some functions are moved to `Configurations`.

#### Additional references

[MAGN-4809](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4809).
[MAGN-4811](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4811).
[MAGN-4807](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-4807) partially.

This PR should be merged after https://github.com/Benglin/Dynamo/pull/93.

#### Reviewers

@aosyatnik, @Benglin, please, take a look. 
Sorry for my previous assessment, if this method ```AddMemberToGroup``` can only handle ```NodeSearchElement```, then we should make it take ```NodeSearchElement```, that should help us catch issues at compile time rather than run time. Sorry about that!