Added support of kerning to Text. Added example for text rendering.

[K-Be]

Now Text does not support kerning for a string. I've adde property kern to support kerning.
Also added example of text rendering to mac and iOS example applications.

[ystrot]

Hi Andrew,

This looks like a good feature! Some quick notes:

• Let's use kerning name for this property, like it named in SVG: https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/kerning.
• Please use SwiftLint to avoid code styling warnings.
• It would be great if you could check why build was failed.

[K-Be]

I'll recreate pull request later.

[K-Be]

Build was failed because some tests failed, I did not look at tests.