functions for notifying on panics

Conflicts:
	yeller.go
add http notification functions
notify url if it's there
gofmt
expand README note to cover full http handler example
put url handling code together
neaten by creating a pointer at first

rather than casting it later
error handling

provides an interface: `YellerErrorHandler` for error handling.

There are two types of errors, and they are logged in different places:
1. Authentication errors, in which the http api responds with a 401
   these are logged straight away, and skip the usual retrying, because
   they'll fail straight away
2. IO related errors/non 200 http codes (all treated the same)
   these are logged once retrying has failed completely

Conflicts:
	TODO.markdown
indentation, and fixing incorrect function names
rename StartWithErrorHandlerEnvironment for consistency