Friend list and formatting

## Friend list
add contact to friend list
modify contact info needed to be finished
load/save friend list as a file
post/fetch encrypted friend list

### REMINDER: look up how to check whether a entry exits in MySQL

## Other changes and formatting
Change `keys\<account>.key` structure to `accounts\<account>\key.dat`
Add `unhex` function and move `tohex` function to the new module `hexparse.py`
other formatting and minor change