Added shorter commands, xp2em and em2xp.

Moved some code to make onCommand() easier to read.
Conversion can now be restricted to one-way.

Added sub-permissions .xte and .etx to xptoemerald.convert. Set to
false to disallow conversion in that direction (using commands).
Signs always allow conversion, so be careful when you place them!
Implemented customization of what value should be used for conversion.

When converting XP to Emeralds, the amount may now specify the amount
of XP, or the number of Emeralds desired. When converting from Emeralds
to XP, the amount may be the number of Emeralds, or the target
experience level that is desired. Both defined by respective options
in config file.
Bug fix: when targeting a specific level, account for current XP.

Yep.
Bug fix: better way to track player's XP (see previous commit).

Now based off of player's current level, in case some progress to
next throws things off.