Check for ember-cli updates and display message to user if an update is available

Add ember update command and change update message to suggest running that rather than npm install

Persist checkForUpdates setting and version information via .ember-cli file

Add .ember-cli files to blueprint tests/fixture directories where needed

Wrap checkForUpdates call/command.validateAndRun() in a Promise to prevent async weirdness

Remove unnecessary ember-cli file (already there)

Make an HTTP call to npm when checking for updates rather than loading npm module

Syntax fix and remove stray call in test to create ember-cli file (not needed)

Add test to confirm update command is run

Update the package.json file manually after doing a global install, so
the correct ember-cli version is reflected there

Add a couple unit tests for update command/update task

fix syntax errors

Remove debug statement

Add entry to CHANGELOG
Change usage of _this to use bind() instead // Remove unnecessary promise // Use .catch() instead of then(success, failure) syntax // Fix some 80+ char lines
Use Promise.denodeify instead of npm functions
Merge pull request #899 from andycrum/check-for-updates

Display message to user if an ember-cli update is available (and add `ember update` command)