Changed PowerSystem to a class instead of an enum

This is for possible expansion further down the road, so I figured it'd
be best to get it out of the way now before I add anything else to make
it even harder to expand.
Fix exception when loading IC2/UE converters
Added charger block

Also now enables BC power if TE installed and not BC, and adds TE recipe
for steam consumer.
Misc fixes

Tried to protect against negative bridge charges
Charger aborts after first item charged so power system should update
correctly
Another pass at making IC2/UE load properly
Removed UE charger support

Because it doesn't actually work. UE's API doesn't document things well
enough to determine how to fix this, so unless somebody submits a PR to
fix this thing, it's going to just sit here unusued and buggy.
Re-added UE charger support

Entirely new code, now works.
Also changed UE converter recipes
Fix IC2 blocks not being removed properly

Also made bridge clamp to 100% charge in case something goes wrong
2.1.0

Added creative tab
2.1.1

Fix NEI exceptions if partner mods are missing
Fix IC2 producers producing energy while not connected to enet
Fix BC consumers not accepting power from combustion engines