It finally works now
Definitely some clean up to be done

Currently writes IHDR and IEND chunks correctly
CRC implementation is borrowed from the specification
Writer class now also has a buffer for the CRC calculation

Also moved both the reader and writer class to the same files, named
files.{cpp,h}

Rows in a bitmap are now padded to be a multiple of 4 bytes long

Implemented padding scalines to 4 bytes if they aren't already that

Also improved error handling slightly

This is so I can test PNG writing by reading a BMP and writing it to a
PNG, effectively the reverse of the PNG reading testing

Currently not incredibly well implemented, but it can handle fixed
codes

The file reader class can now support either endianness to read in,
and will convert to the native endianness