Fixes #6202: allows setting the color and style button attributes at once.

[iignatov]

Short description of what this resolves:

Allows setting the color and style button attributes at once and ensures that they are visualized as they are set in the code (as shown in this plunker demo).

Changes proposed in this pull request:

• Correctly resets the style to its default value.
• Adds array support to the color attribute which is also supported via multiple attributes.
• Correctly resets the color/style classes when updating color and style attributes.

Ionic Version: 2.x

Fixes: #6202

[brandyscarney]

Thanks for the PR! :)