Deprecate old parsing infrastructure and provide new entry points

[jrudolph]

The main entry points for parsing are now

• spray.json.parseJsonAs -> parse JSON document to Scala object T directly
• spray.json.parseJson -> parse JSON document to AST

There are also enrichments for the supported parser input types.

Inputs themselves are now closed down to String, Array[Char], Array[Byte], and a more generic RandomAccessBytes that can be implemented by third parties.

Internally, we still use the current JsonParser infrastructure but that will go (or not be used by default any more) in the future.

[jrudolph]

I changed the default branch in Github to release-1.3.x for now, to be sure not to clutter the README with irrelevant information.