Adds Category (and tag) Mapping

Notes on the patch:
Simplepie doesn't pull attributes, so there is no real way to
differentiate currently between categories and tags. My solution was to
check first for existing categories, if not exist, tags, if not exist
create as a category.
Code Stylistic Change

Reversed the condition to prevent a negative condition.
Whitespacing

Changed whitespace to match orig.
Logic fix

Removing negative conditions.
Logic fix

Replaces nested conditions with if else if
whitespace

whitespace fixed
Merge pull request #43 from Jackreichert/master

Adds Category (and tag) Mapping