implement :abbreviate and :unabbreviate
edit changelog
clean up
Added tag 2.8.0 for changeset 4257e6401241
edit readme
fix #446 - improve I and A with visual selections
#437 - fix path for unix
move files

--HG--
rename : abbrev.py => vi/abbrev.py
fix #450, #451 - improve gg and G
fail
Fix repeat of insert

Fixed the issue that repeat by '.' after inserting characters by 'i'
doesn't work.