reorganize code

move all code to / and have only a main package. don't bother making up packages, we never gonna use them anywhere else.
lots of things
list: print emails while reading them

don't read all emails and print them, but read all filenames and parse/print emails as we go.
make account a *Mailbox; small fixes to code and readme
use int64 instead of string for msg id - nmh is about numbered files; improve mh.Append
simplify some code; use Message instead of *Message, this struct is small
throw away interfaces; mailbox will be a nmh mailbox client only
implement edit case in main; other small fixes
grep msg body
rename 'pick' to 'grep'
improved pick: now its possible to specify 1..n header keys for searching
mail is moved to sent folder after sending
fix attach
add MIME-Version to mime header
write \n instead of \r\n in email header, seems that the rfc is not followed by anyone
sent first mail with mailbox!
fix package declarations of mh package
simplify save and parts cmd by replacing channels with string slices
rewrote sequences so i can use it for mime parts
lots of small improvements