Make flycheck-pos-tip-hide-messages work without minor mode

[bcc32]

This way, the package works as expected even when the user enables it by customizing flycheck-display-errors-function (or happens to save such a customization permanently after enabling flycheck-pos-tip-mode).

Fix #36.

The new implementation strategy is to add flycheck-pos-tip-hide-messages to the appropriate hooks temporarily when the display function is called, and remove it when the messages are actually dismissed. This avoids depending on the minor mode being enabled at all.

Now, the only job of the minor mode is to be able to interactively toggle the value of flycheck-display-errors-function.

[cpitclaudel]

Woops, I missed this PR. It looks good to me, I left one comment