Check for null when comparing using the most precise type. (#16)

* Check for null when comparing using the most precise type. This allows null value parameters to be used in expressions such as "'a string' == null" where null is defined as e.Parameters["null"] = null.

* Added AllowNullParameter option to define a null parameter and allow comparison of values to null. Set as an option to not affecting existing use of NCalc.

* Added some additional null checks from #3

* Remove .GetType() call from loop, fix typo.

* Removed unused overload.

Merge pull request #15 from spudstuff/master

Improve performance for release builds by removing TRACE constant.

added net35 folder