Login is case sensitive

[Martijn Luinstra]

Make email field at login case insensitive, as some people register with upper-case characters in their email (e.g. [e-mail]) instead and try to login in lower-case. As most email providers don't support case sensitive email addresses, it makes more sense.

[Jelmer van der Linde]

Do not take case into account in DataModelMember::login. This fixes issue #52.

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