Added a :gc command

This exposes a means to trigger a :gc in the vm hosting the repl. This is useful for long running sessions, and the benefit can be seen by doing the following:

Start java REPL, connect to the VM with jconsole or another monitoring tool of your choice, watch the memory usage in particular.

In the REPL:
```
java> import java.util.*
```
Now watch as the memory usage spikes. Then,
```
java> :gc
Performing GC...
```
Behold your newly freed memory!