Add Vim 7.2 support

Added backwards-compatible replacement functions for settabvar and
gettabvar using setwinvar, settabwinvar and gettabwinvar. Each window
in a tab contains the 'tab variable', rather than the tab itself.
Requires syncing on creation of new windows.
Refactored to for simplicity

Use global tab-number-prefixed variables instead of tab variables
Minor refactoring
Merge pull request #8 from shinymayhem/master

Vim 7.2 support