Fix the email regexp CPU usage for odd inputs

Fixes #34

Some inputs could make the regexp engine in Ruby take up a lot of CPU time in order to check for a match of an email in the input 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.

The new email regexp is based on the [changes suggested](https://github.com/tenderlove/rails_autolink/issues/34#issuecomment-26795839) by @andresbravog in #34 but is changed a bit so that tests pass.