Fix input transformer bug when pasting ipython promts (issue #6674).

Fixes a SyntaxError that arose when pasting code containing the IPython
continuation prompt without preceding spaces, eg.:

In [1]: if True:
...:     print "No spaces before the '...:' prompt."

This issue was brought up in
"http://stackoverflow.com/questions/26297356/how-can-i-paste-the-ipython-output-in-ipython#comment41270724_26297356"

This commit adds a test for the bug, and fixes the regular expression
that strips continuation prompts.
Do you want to remove the mention of #6674 here?