Add subtitle to ApsAlert

[dsanghan]

Tested for APNS. Works.

[googlebot]

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[hiranya911]

@dsanghan why was this closed?

[dsanghan]

I closed the issue and opened the PR

[googlebot]

CLAs look good, thanks!

[hiranya911]

@dsanghan Can you fix the linter error detected by Travis (line-too-long)? Also please update the tests.

[hiranya911]

Closing in favor of #239