Adds WalkerSelector that picks appropriate walker

It selects a DocumentWalker appropriate for the language of the document
Supports .vb: Adds VBWalker and CSWalker

CSWalker is DocumentWalker, renamed.
Allows displaying .vb HTML files
Adds reference to VisualBasic.Workspaces

so that we can load the vb projects
Merge pull request #47 from AmadeusW/vbSupport

Closes #44: Adds VisualBasic Support