Improve MacAddrField

MAC addresses are now displayed with colons. They're still stored in the
database without colons.
Update MacAddrField usage
Fix some capitalization issues
Don't use mac_str; remove it
Use mac, not mac_str
Merge pull request #606 from drkitty/macaddrfield-fixes

MacAddrField improvements, fully functional, does what its supposed to do. Merging.