Represeting stack-traces better

[rodedzats]

Now we represent stack trace as a simple string. We need to create a stack-trace object which will contain more informative details about the stack trace, such as the type of the exception and a list of all the error sentences. After that we need to change the already implemented algorithms to use this entity.

To summarize we need to:

• [x] Create a stack trace object which will contain more info.
• [x] Change existing algorithms to use this entity

[yonzarecki]

Looking for places where Strings are used instead of the "StackTrace" entity