Check buffer existence in s:ExtraditeIsActiveInTab

This prevents errors when buffers are closed incorrectly (e.g with
:q command).
Invert if condition at the top of s:ExtraditeClose
Don't return to original buffer for :Extradite!

After closing it.
Remove main Extradite's buffer on close
Merge branch 'fix-split-close'

Resolves #28.