Copy public headers to built products dir for iOS target

This allows the use of CocoaLumberjack as a subproject in cases
where Cocoapods is not desired or possible.
put pre-processor macro at beginning of file

also unified the macro style
Merge pull request #378 from DivineDominion/patch-1

put pre-processor macro at beginning of file
Merge pull request #379 from bloom/subproject-support

Copy public headers to built products dir for iOS target