Implemented several new commands and moved the download location to the temp folder. ...

- dub install installs a package in the user/system repository or in the current directory
 - dub uninstall is not yet implemented
 - dub add-local adds a local folder as an installed package (useful so that projects use a git cloned package instead of the github one)
 - dub remove-local removes a local folder again as an installed package

Implement basic support for user/system installed packages and local packages. ...

Also removed a lot of (caught) exceptions. The installation/uninstallation code was moved and modified to the new package manager, which is now responsible for all locally installed packages.