Caching buildtrees

Combination of: #311 and #21 (closed)

Introducing caching of build trees

• cache-build-tree var has been added to the project.conf defaults (defaulting to True)
• When cache-build-tree is set to True, the contents of the sandbox build directory is copied to the cache
• The cache key dict has been updated to reflect the status of cache-build-tree

Original commit required additional functionality to test

"Opening a workspace with a cached build"

• app.py: modified Open_workspace to allow use of cached build tree
• cli.py: added additional flag to workspace open, workspace reset, build, pull
• element.py: Added function to return cached build tree dir

Cache is too big

• Limited the git history to reduce cache size
• omitted .bzr directory at the point of staging
• Only downloads from remote cache on demand