Add new API function ihex_rs_get_address_range().

The record set is scanned for its minimum address and the maximum
address plus associated content length.  These two parameters can then
be used to allocate a suitably sized range for copying the data.
Fix: Include record length in maximum address calculation.
Merge pull request #18 from acolomb/get-address-range

Add new API function ihex_get_address_range()