Don't show 'try %paste' message while using magics

Fixes #1661.
Minor updates to reflect comments
fixed variable name change
added test_paste_magics_message
Added try/finally block to set using_paste_magics
Added test for TerminalMagics
Merge pull request #2744 from mrshu/magics_fix

Don't show 'try %paste' message while using magics