Fix FSharp.Core resolution issue in the MonoDevelop add-in

The add-in has to resolve mscorlib and FSharp.Core using the
project assembly context.
Fix incorrect assignment in last commit
Added Restart/Clear commands and cleaned up Interactive commands
Fixed typo
Update README.md
Ensure framework version is considered, rather than hard-coding to 4.0
Updated for F# 3.1 and .Net 4.5 framework versions
Ensure framework version is considered in ResoverProvider
bump version and remove 3second pause
Implemented last window caching with a single instance of the tooltip window

Also added a failsafe on displaying empty tooltips
Cached last tooltiptext
Cleaned up the code slightly
Switched to mutable rather then ref