Allow email address to be collected within contribution profiles

Currently, if an email address field is included in a profile (I've only tested with primary), and that profile is included in a contribution page, the email address field is stripped out of the profile, as it it collected at the top of the contribution page under the amount box by default. The contribution pages should detect the presence of a primary email field in an included profile and use that instead of the default if it has been provided. This would give he flexibility to locate the email address field in places other than the top of the form where if often doesn't make sense from an end user point of view.