Clarify the licensing

[ForNeVeR]

• Create new Licensing.md file and link it from Readme
• Remove (or clarify) my additions from LICENSE.txt
• Seek any additional licenses (e.g. one used in ResTool, alternate authors for netlib.Dns, some Unicode stuff in stringprep, notice in xpnet) and enlist them in Licensing.md
• Accurately describe the whole picture in Licensing.md
• Update copyright notices in the relevant files (AssemblyInfo and jabber-net.nuspec)
• Explain the source updating policy in Licensing.md (e.g. that we aren't changing source headers and not introducing them in the new files)
• Update license tags (for the last time I hope)