Fixes gh-611, gh-612 - Add kill count system and kill meters with XP.

The killstreak counter shows a message if the player has more than 5
kills. I have also added an example reward system with points, but did not
enable it yet (needs discussion).

Through the warzone option xpkillmeter the XP level will get updated with
your current kill streak amount. This gets reset every time you die and
is added to every time you kill a player. If this option is enabled,
players cannot gain XP in the warzone.
Add base for configurable KS rewards. Fixes gh-506

Killstreak rewards can be configured in the set.war.killstreak section of
war.yml. Rewards will only be applied to players if the team/warzone has
KILLSTREAK set to true.

The configuration section will get populated on generation of war.yml, or
may be manually added to older configurations.

Currently the system supports messaging the entire warzone and just the
attacker. It only supports rewards of items and health at the moment; more
are expected to arrive.

This commit also moves XPKILLMETER into the team-specific section of the
config.
Allow the use of multiple items in rewards.

This also allows the use of XP and points as rewards.
Added configurable airstrikes, based on work from @grinning.
Add MySQL kill/death logging support. Closes gh-658

War now logs kills and deaths to a MySQL database. Records are created for
every player at the end of each round with the current date and amount of
times the player killed and was killed for the entire round. It has built
in support for automatic & configurable log clearing past a certain date
(default is 1 week).
Handle deaths after they have been counted and rewarded.

This prevents a problem where players could be rewarded after the game ends
Fix issues with joining a team.

Players are no longer silently kicked out of their team if they try to join
a non-existant team with /join. Fix an inconsistency with the format of the
team join notification message.