Implement async upgrade checker

* Reads from github looking for updated tags
* Runs in a goroutine to not slow down normal operation
* Uses the timelock cache to only check github at intervals
Add documentation for upgrade checks
upgrade: Fixes and cleanups from pull request
add: Cleanup and bug fix

* Remove ability to add a second dep since that never worked anyway
* Remove promptBool (newly unused)
* Update Documentation
General cleanup

* Fix golint problems
* Expand makefile to run depman with various flags
* Makefile should test using golint
Bump Copyright year
Run goimports
Reorganize documentation to support Github and Godoc.org

* Move Readme content into doc.go
* Edit and cleanup documentation
* 'make doc' will now generate Readme.md from doc.go
* Add godocdown as dependency
* Update dependencies to latest versions
Merge branch 'master' into feature-cleanup

* master:
  upgrade: Fixes and cleanups from pull request
  Add documentation for upgrade checks
  Implement async upgrade checker

Conflicts:
	Readme.md
upgrade: fix golint warning
Bump version to 2.9.1
Merge pull request #27 from vube/feature-cleanup

Feature cleanup
Fix show-frozen command for mercurial repos

* Version 2.9.2
timelock: Change clear cache message to yellow
upgrade: Simplify upgrade code

* Don't notify user of new version while upgrading
documentation: Add info about --skip-cache and source location
Bump version to 2.9.3
upgrade: Run the new version of depman after upgrade

* Inform the user if a new version was found or not
* Bump version number