Stop referring to "installation" in commands

Changes commands names, help output and actual code to use more neutral
`get`/`remove` and derivatives. Old command names remain undocumented and
warned about.

This is somewhat temporary solution to avoid common user confusion in the short term.
Having a quick run through the source during renaming I really got an impression that dub
should benefit a lot from creating a clear definitions / vocabulary for used entities and
providing more hirearchical commands endorsing those. This is something I am not going to
speculate above until registry enhancements though.