New command `rm` (`remove`, `delete`, `del`) allows
to remove one or more collections and resources from an eXist-db
instance.

Special paths, like `/db` and `/db/system` are protected and cannot be
deleted as they render the entire database unusable.

Examples:

Remove two resources

```
xst rm /db/temp/a.xml /db/temp/b.xml
```

Remove an entire collection

```
xst rm -rf /db/apps/my-app
```

Passing `-` as the positional parameter to `xst exec` reads the the
query from standard input.

`echo '1+1' | xst exec -`

Add tests for reading b, f and query from stdin

This allows to download resources and collections to the filesystem.

- Remove temporary collection for packages after installation.
  The collection is removed regardless if the installation succeeded or
  not.
- bump node-exist to v5.0.1 (to access packageCollection value)
- add test to check if cleanup was successful
- clean up install tests from old cruft

BREAKING CHANGE:
`xst install` is now  `xst package install` to prepare for
`xst package list` and other package related subcommands.
This is to avoid breaking changes in the near future when package list
is added  as `xst list` is already taken.

closes eXist-db#34