Detailed debugging.

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)
More detailed debugging.

Moved logging functions to autoload/syntastic/log.vim.
Cleanup debug functions.
Add a function to dump option variables.
Merge branch 'debug'