Removed the vibe.d dependency by dragging in a minimal set of interfaces of vibe.d ...

The interfaces are reimplemented using standard Phobos functionality. Calls to
these interfaces will be gradually replaced by direct Phobos calls.

Avoid use of deprecated functions + fixed package installation.

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.

Compiles using latest vibe.d and DMD 2.061.

Initial commit (more or less the same code as used for vibe.d's vpm).