Finish language services #287

While e.g. the current intellisense is functional, there's room for improvements just based off the existing racer implementation.

• Provide original paths to racer for better static autocompletion (based on Cargo.toml?)
  See MaulingMonkey@7649846

• Use racer complete-with-snippet ... to get and embed documentation into the signature provider
  See MaulingMonkey@dd67195
  & MaulingMonkey@9675cdd
  I can't figure out the API(s) (if any) to get keyword coloring / monospaced fonts etc. however.
  Example of the above two changes:

• Provide signature/parameters support
  Testing v1 available in my branch: https://github.com/MaulingMonkey/VisualRust/tree/sighelp
  Ref: https://msdn.microsoft.com/en-us/library/ee334194.aspx

• Provide quickinfo hovering tooltips
  Ref: https://msdn.microsoft.com/en-us/library/ee197646.aspx

• Implement outlining
  This could perhaps reuse existing AST stuff used for syntax highlighting
  Ref: https://msdn.microsoft.com/en-us/library/ee197665.aspx