Add tests for the update command

[wibblymat]

To catch issues like #282

[necolas]

@satazor Is update only meant to update to the latest patch release when you've specified a semver with explicit major and minor? It's not a very clear what this command is for.

[satazor]

@necolas both install and update commands are ambiguous as they stand now.

Will quote something that I emailed in the last bower meeting:

• When executing bower install, everything is installed every time. There is no check for already installed packages. Also, when installing a specific package, its dependencies might override previously installed packages, even if the version required by other project packages is not compatible, potentially breaking the project. This is directly implicated by the current implementation of the Manager. Warn if existing component is upgraded silently as part of another component's install #200
• As for bower update, it's almost the same as running a fresh install. It will fetch and install everything once again, making it a slow process. Bower update of a single package updates all #256. The same issue as install (Warn if existing component is upgraded silently as part of another component's install #200) occurs when updating specific packages.

The update command updates to the latest patch release, which is kind of incorrect if the user specified a major or minor range.

[necolas]

Thanks @satazor. Looking at the functionality of update in other package managers, Bower's felt confusing. Good to know that it actually is confusing!

[satazor]

The update on the rewrite works great now! Also tests are in the todo list!