mdCard: img selector should be more specific to avoid breaking custom layouts

[epelc]

Actual behavior:

• What is the issue? * df902381 seriously breaks any custom layouts using cards and images. The selector should be changed and possibly remove !important but more so the selector changed.
• What is the expected behavior? The selector should target md-card-header img or add a class(maybe .md-img to allow for custom layouts. ping @ThomasBurleson

CodePen or Steps to reproduce the issue: *

• CodePen Demo which shows your issue :
• Details:

Angular Versions: *

• Angular Version: 1.5.6
• Angular Material Version: 1.1.0-rc.5

Additional Information:

• Browser Type: * chrome
• Browser Version: *51.0.2704.84 (64-bit)
• OS: * os x 10.11.5
• Stack Traces:

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