Create an overall MemoryManager class

[NickDraper]

This class will be a central point of organization for our memory management efforts.

It will need to be able to
- Determine the amount of memory available for both windows and linux
- Determine the amount of memory used by Mantid data stores (primarily the analysis data service)
- It will be used by the WorkspaceFactory to determine whether to issue a memory reduced version of an algorithm.
- It may (later on) inform less recently used workspaces to save themselves to disk to reduce memory.

[NickDraper]

This issue was originally trac ticket 189