Add "class/" to .gitignore

The class directory contains pre-compiled classes.
Add LogItPlugin.getInstance() method

.getInstance() will return current LogItPlugin instance from
org.bukkit.plugin.PluginManager.
Add LogItPlugin.loadLibrary(String) method

.loadLibrary(String) will load requested library. In case of exception
it will log the failure, disable LogIt and rethrow the exception.
Load mail.jar in MailSender's static initializer
Load h2.jar in H2Database's static initializer

Also, remove old code that downloaded and loaded h2.jar from the internet.
Remove all library-oriented FileUtils calls from LogItCore
Check if mailSender == null in LogItCore.sendPasswordRecoveryMail()

If mailSender is null, throw RuntimeException with appropriate
message.
Use LogItPlugin.getInstance() instead of PluginManager.getPlugin()