Obsoleted `BeBlank` and `NotBeBlank`.  Added `NotBeNullOrWhitespace` and `NotBeNullOrWhitespace` to fix issue #7
Updated StringAssertionSpecs to no longer use `BeBlank`, `NotBeBlank`.  Update affected method's names to newer convention.
Capitalized the 's' in Whitespace to make the method consistent with the `String.IsNullOrWhiteSpace` method
Merge pull request #40 from vossad01/Issue#7

Added `BeNullOrWhiteSpace` and `NotBeNullOrWhiteSpace` methods to StringAssertions.