Switch to using a compiled library

Instead of using raw C# code in the DNA file, this version
uses a compiled library (dll). This lib is packed into a the
xll via ExcelDnaPack.

Also added UnitTests

Also added a Visual Studio Project for easier testing/dev
Upgrade regex to detect valid emails
Add .dna file to sln
Minor code cleanup
Add new method (untested) DNS.Resolve

And renamed the libExcelRegex directory to lib
Add DNS.GetMX(domain)

Returns MX record(s) for a domain