XML-RPC: Initial implementation of Taxonomy and Term APIs.

Implements: wp.newTerm, wp.editTerm, wp.deleteTerm, wp.getTerm, wp.getTerms, wp.getTaxonomy and wp.getTaxonomies

See: #18438, #18439, #18440, #18441, #18442, #18443, and #18444 props maxcutler, markoheijnen and nprasath002.


git-svn-id: https://core.svn.wordpress.org/trunk@20137 1a063a9b-81f0-0310-95a4-ce76da25c4cd