Added ability to set the background-color of the spinner backdrop.

[krishnakumarp]

Implements the ability to set the background-color of the spinner backdrop. Refer #143

[coveralls]

Pull Request Test Coverage Report for Build 385724076

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 362358949:	0.0%
Covered Lines:	91
Relevant Lines:	91

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💛 - Coveralls

[mpalourdio]

Thanks for your contribution, I have just tested and there is a problem now :) When the backdrop is disabled ([backdrop]="false"), there's now a default background color behind the spinner. Will think about it a bit later.

[mpalourdio]

I have amended your PR with some commits. Merging now, will release later! thanks a lot for contributing!

[krishnakumarp]

Thanks!