Write Concepts wiki documentation

[NickDraper]

This is to explain the major concepts of Mantid to users and algorithm developers.

Todo
- instruments and geometry
- An overview of the roles of the different parts of the framework
- properties
- analysis data service
- An explanation of how you should interact with the framework as a user (i.e. via the MantidAPI from Algorithms) and how to create, hold and manipulate references to workspaces and properties
- Some examples of doing all the above

Done
- Algorithms
- workspaces
- dynamic factories and plugins
- FrameworkManager
- shared pointers

[NickDraper]

This issue was originally trac ticket 173