Adding unit test infrastructure and some actual tests

Here's the changes as discussed in issue https://github.com/mjsabby/RoslynClrHeapAllocationAnalyzer/issues/1#issuecomment-59076880

Basically I've done the following:
- https://github.com/mattwarren/RoslynClrHeapAllocationAnalyzer/commit/a51ca5391c3fa5fe00933976cd21f1524260c568 - added some unit testing infrastructure and tests for the following Analysers:
  - ConcatenationAllocation
  - EnumeratorAllocation
  - ExplicitAllocation
- https://github.com/mattwarren/RoslynClrHeapAllocationAnalyzer/commit/0f9f62fda6fee5c8ccbde5b644fb1c86d599f24d - added support for a few more scenarios and fixed some bugs in:
  - EnumeratorAllocationAnalyzer.cs
  - ExplicitAllocationAnalyzer.cs