Simplified code to deal with buffers only

This commit simplifies the library to only offer the ability to save/load
encrypted tox messengers to buffer, fixing issue #4.
Edited README.md to reflect changes
Edited README.md
Minor fixes to tox_data.c
Fixed mistake made in memcpy() calls

This commit fixes an error introduced when retrofitting calls from fread() to
memcpy() - dest and source were switched pretty much universally (ha!).
Comment fix
Edited tox_data.c to behave better