Allow site enabledFrom and enabledTo values to also be null instead of requiring DateTime values only.
Update getting_started.rst

Better Markdown formatting for field description section under 'Creating a default site'.
Update getting_started.rst

Updated table format to use reStructuredText Markup.
Merge pull request #266 from SonyATV/sonyatv-2.3.x

Allow site enabledFrom and enabledTo dates to be null; Added site selector unit tests.