Fix the email regexp CPU usage for odd inputs

Fixes #34

Some inputs could make the Regexp engine in Ruby to take up a lot of
CPU time in order to check for a match of the email string. This change
adds a test to guard against an example of such an input and changes
the email regexp so that all the tests pass and so that no excessive
CPU usage occurs.
Merge pull request #36 from mitio/fix-greedy-email-regexp

Fix the email regexp CPU usage for odd inputs