Refactored IEnumerable<> calls to use 'yield return' for streaming

[aaaaaandrew]

Refactored IEnumerable<> calls to use 'yield return' for streaming data vs caching it all in a List<> and then returning it. This is the only way to use LinqToExcel w/ large files (> 1gb or so).

[paulyoder]

Hi @amattie, When I run the test suite against your pull request, there are over 70 failures. I wonder what is causing the failures.

You can run the test suite by double clicking the tools/test.bat file.

Let me know if you're able to fix the tests, and I'll be happy to merge in your changes.