ENH: Create session for current tab page only.

Depending on whether 'sessionoptions' contains "tabpages", GUI, font, features, colors, fullscreen are not persisted. Together with new :SaveTabSession, ... commands (only defined when the default 'sessionoptions' persist all tabs; otherwise, the original commands are enough and now work on single tab pages), this allows a more lightweight form of persistence limited to a single tab page. When such a single-tab session is restored, it also only deletes the buffers that were solely visible in the current tab page, and it replaces only the windows in that tab page. Additionally, a new :AppendTabSession command allows to add the session as a new tab page.

Public API functions xolox#session#is_tabsession() and xolox#session#session_name() have been added for use e.g. in 'titlestring'.

A limitation of the current implementation is that multiple (tab-only and/or global) sessions cannot be handled concurrently; due to the script-local variables, there's only a single notion of "current session", but that shouldn't matter that much in practice.
Avoid empty session name in auto-save query.
Merge pull request #35: Create session for current tab page only

I ported Ingo's code (based on vim-session 1.5) to vim-session
1.8 and changed quite a few things in the process:

 - Most significantly, in my version :OpenSession, :SaveSession
   and :CloseSession keep their original meaning and the commands
   :OpenTabSession, :SaveTabSession and :CloseTabSession should be
   used to manage tab scoped sessions.
 - Added documentation
 - Made periodic auto-save respect tab scoped sessions
 - Made lock files aware of tab scoped sessions and process IDs
 - Replaced set_session_name() with t:this_session / v:this_session
 - Auto-load functions for tab scoped commands,
   for easy re-use in command aliases
 - Renamed PushTabSessionOptions() to change_tab_options()
 - Renamed PopTabSessionOptions() to restore_tab_options()
 - Reimplemented session_name() as find_current_session()
 - Reimplemented is_tabsession() as is_tab_scoped()
 - xolox#session#save_session() now rewrites 'doautoall'
   in tab scoped sessions to 'windo doautocmd'