Log a warning when a theme is not loaded

[jelbourn]

Not having a theme loaded is the most common issue that people seem to encounter. We should find a way to throw a dev-mode warning when we detect that a theme isn't loaded, similar to what we do for HammerJS not being loaded.

Ideally we would avoid using getComputedStyle or anything else expensive. We can add whatever simple marker style to the md-core mixin necessary if that helps.

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