Add a feature in JsonParser to verify that no duplicate field values are given

[cowtowncoder]

Although JSON specification does not make duplicate Object values strictly illegal (behavior is undefined I think), their existence is problematic, as behavior is not specified.
On the other hand, checking for duplicates adds non-trivial amount of cost which for valid content is pure overhead.

So it would probably make sense to add a feature to let users request that duplicate detection is enabled, and reported as an exception. For bonus points it should be possible to dynamically enable this via ObjectMapper (from databind) as well, since this will allow per-request handling (as opposed to JsonParser feature which may be more difficult to set).

[cowtowncoder]

Started working on this, should make it in 2.3.0.

[cowtowncoder]

Implemented; along with #106.