[WebReferences] Fix typo in add-in description

"maintianing" -> "maintaining"
[AssemblyBrowser] Nitpick in add-in description

"for the MonoDevelop" -> "for MonoDevelop" sounds better
[WebReferences] Nitpick in add-in description.

Languages are better referred as "C#" and "VB.NET" instead of
"CSharp" and "VB".
Merge pull request #352 from knocte/description_nitpicks

Add-in description typos and nitpicks