Develop macro refactoring

[bvdmitri]

This pull request is a part of #82 issue:

Pull request covers some macro source-code, fixes anti-patterns and is aimed at improving macro code readability and robustness.

1. @RV:
   • macro definition has been split into several functions with different logic
   • macro definition and tests have been moved from probability_distribution.jl to variable.jl
   • tests added for unsupported usage of @RV macro
2. @symmetrical:
   • macro definition has been reimplemented from scratch
   • evals have been removed
   • support for multiple where blocks
   • fixes out-of-scope error while using this macro outside ForneyLab main module (e.g. experimental nodes which are defined in other modules)
   • tests added
3. *Rule macros:
   • strings manipulations have been replaced with Julia expressions
4. @ensureVariables:
   • strings manipulations have been replaced with Julia expressions

[ThijsvdLaar]

Thanks a lot, I love it, especially the map and reduce in the rule validation.

[ivan-bocharov]

Thanks for the PR @bvdmitri. I think it can be merged into master.