Jackson upgrade to 2.1.2

This patch upgrades Jackson all the way from 1.8.5 (released mid-2011) to 2.1.2 (released December 2012). Depending on this quite old version of Jackson has been making some recent work with Wiremock very inconvenient, so I've fixed it.

There's are very few substantial changes required outside of import renaming, with one notable exclusion: upgrading past 1.9.0 made ResponseDefinition hit [JACKSON-783](http://jira.codehaus.org/browse/JACKSON-783). This bug/feature means if you have two overloaded methods for the same property (here `setBody(String)` & `setBody(byte[])`), and you mark one as ignored, both are ignored, which differs from pre-1.9 behaviour, and breaks _everything_. setBody(String) is now explicitly marked as a `@JsonProperty`, thereby solving this.