New variables: g:syntastic_debug_file, the name of a file where to write
debugging messages, in addition to adding them to the message history.
The old g:syntastic_debug is now a sum of flags:
* 1 - trace checker calls
* 2 - dump loclists
* 4 - trace notifiers
* 8 - trace autocommands
* 16 - dump syntastic variables (not implemented yet)
(1) Checkers now have an _exec attribute, and an accessor getExec().
(2) CreateAndRegisterChecker() initializes _exec from an optional argument
'exec'. If this argument is missing, 'name' is used instead.
(3) Functions SyntaxCheckers_*_IsAvailable() are now dictionary functions.
(4) Functions SyntaxCheckers_*_IsAvailable() are now optional. When
they are missing, they are assumed to return executable(expand(self.getExec())).
(5) Argument 'exe' of function syntastic#makeprg#build() is now optional.
If this argument is missing, expand(self.getExec()) is used to set checker
executables.
Make SyntaxCheckers_*_GetLocList() dictionary functions.
Pass a reference to the current checker to syntastic#makeprg#build().
Add an optional 'redirect' argument to CreateAndRegisterChecker().
Change the sh checker to use the new dictionary functions.
Add a new registry method getLocListRaw() (needed for the sh checker).
This is not completely safe: if a loclist is created by something
other than syntastic before the error window is displayed, syntastic
will obliterate it. There is currently no way to tell if a
loclist was created by syntastic or not. Undocumented variable
g:syntastic_reuse_loc_lists can be used to disable reuse of loclists.
Also handle the case when user disables notifiers after the first run.
This doesn't work for signs though, since it causes an ugly flicker in
the common case.
