do refactoring for the info window

move functions related to the info window to
autoload/grammarous/info_win.vim.
this is for not only common purpose of refactoring,
but also unite source's actions
Merge branch 'issue/5' which fixes issue #5
get bufnr even if the source is opened by :Unite source
execute checker if no check is executed in the buffer yet
implement 'grammarous' unite source's actions
tiny fixes

- fix doc for 'remove error' feature
- fix fixit()'s arguments
make sure to check grammar on starting 'grammarous' unite source
about unite source for grammarous
close the info win before open unite interface for grammarous source
Merge branch 'issue/3' which fixes issue #3
be quiet on deleting highlight
make commands be enable to be followed by another command
reset all before grammar check
some document fixes and reinforcement