Cuts down Atlas.sol by 238 bytes in total. Needed to squeeze in #456

Changes:

• Made onlyWhenUnlocked an internal function instead of a modifier (cuts 40 bytes)
• Added indexed to appropriate event fields, saves a bit of size per field (cuts 33 bytes)
• Replaced Transfer events with Mint and Burn events (cuts 12 bytes)
• Removed redundant uint112 overflow check, done in SafeCast on line below (cuts 40 bytes)
• Moved SafeCast operation to inline instead of creating new stack var (cuts 8 bytes)
• Refactored _handleErrors in Atlas.sol to save size (cuts 105 bytes)

Most of these will also save a bit of gas.