Bread crumbs are fixed

#### Purpose

Remove top category from bread crumbs. Image to understand what the PR resolves.
![removetopcategoryfrombc](https://cloud.githubusercontent.com/assets/8158551/4884520/86f6eb90-636c-11e4-879e-0244985f102e.PNG)
 
#### Modifications

Modified logic of `SearchMemberGroup.Prefix` property to remove top category from it.

#### Additional references

[MAGN-5225](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5225).

#### Unit tests

Created new test function `SearchMemberGroupTests.PrefixTest`.
![image](https://cloud.githubusercontent.com/assets/8158551/4896708/1db15e08-63fe-11e4-8439-abe79b2e3f0e.png)

#### Reviewers

@aosyatnik, @Benglin, please, take a look.