Adjustments to support ePub2 files

Two fixes here. The first is that the code was expecting the first identifier tag to be the one referenced by the package tag which it may not be. This is not specific to ePub 2 support.

The ePub 2 fix is that ePub 2 does not require the modification date metadata field. I've added a check against the package version before throwing the error. I'm not sure if atof is the proper way to handle comparison with the attribute value. Feel free to alter as necessary.