initial implementation of auto-snapshot when library state mutates
emit client event when restore is needed
check for changes to root library directories
use simple reentrancy guard to avoid parallel execution
render Packrat version with same style as library version
mute all columns in the packages pane
show correct versions in Packrat and library version columns
restart auto-snapshot when library changes occur during snapshotting
don't monitor library mutations for non-Packrat projects
Merge remote-tracking branch 'origin/master' into feature/auto-snapshot

Conflicts:
	src/cpp/session/modules/SessionPackrat.cpp
Merge remote-tracking branch 'origin/master' into feature/auto-snapshot
use pointer for recursion guard
Merge pull request #118 from rstudio/feature/auto-snapshot

Automatic snapshotting for Packrat projects