Rename `TextNode` as `StringNode`; `JsonNode.xxxTextYyy()` (mostly) as `JsonNode.xxxStringYyy()` [JSTEP-3]

[cowtowncoder]

As per JSTEP-3 (and related to JSTEP-6) we want to unify naming of types and accessors: Jackson 2.x uses "Text" instead of "String" (unlike everything else in Jackson 2.x API), so with 3.0 we can fix the discrepancy.

So we will:

1. Rename TextNode as StringNode (it is unlikely actual type often referred to in application code)
2. Rename JsonNode accessor methods to use "String" instead of "Text" in most cases; deprecated+add in others

What this specifically means is that we'll change:

• asText()/asText(String): deprecate, add asString()/asString(String)
• findValuesAsText(): rename as findValuesAsString()
• isTextual(): deprecate, add isString()
• textValue(): rename as stringValue()