swirl_out message when leaving swirl in 3 places in swirl.R: 2 in resume method and 1 in bye()

also replaced call to initial call to bye() in swirl()
 with call to removeTaskCallback so that exit msg in  bye wouldn't be printed
Roxygenized exported functions.
Resolve merge conflict in swirl.R

Merge remote-tracking branch 'gina/master'

Conflicts:
	R/swirl.R