add StringEnumMapper, wip for custom String transformer and ignoreCase()
add ignoreCase for toString mapping + use single static factory method
add default value support + common interface + javadoc

default value support is currently coded only for ByNameEnumMapper
public method and interface have javadoc
split unit tests in several classes for better readibility
add IStringEnumInterface implemented by all mapper created by StringEnumMapper to factorize unit tests