Allow one Extradite buffer per tab page

Rather than only one Extradite for Vim instance.

For this convert g:extradite_bufnr to t:extradite_bufnr and change
all checks to check for the existence of t:extradite_bufnr first.
The later check was put into the ExtraditeIsActiveInTab() function.
Switch to the original buffer rather than quiting

It makes using Extradite with tabs much more convenient.  Tabs are not
closed on leaving Extradite anymore.
Merge pull request #23 from int3/better-tabs-support

Better tabs support

* Allow one Extradite buffer per tab page.
* Also includes a change that prevents tab page from being closed on quiting Extradite buffer.

Fixes #8. Well, implicitly, by switching from global variable to tab-variable.