De-qualify checker symbols

Remove the `flycheck-checker-` prefix from all syntax checkers.  We do
not risk any name collision by doings so, because checker symbols are
not used as functions or variables, but only to attach symbol properties
which are properly qualified.
Attach configuration file variable to checker symbols
Add a getter for the major modes of a checker
Add a command to show help for a checker
Add documentation to checkers
Fix casing
Merge pull request #52 from lunaryorn/checker-documentation

Add documentation to checkers