First CRAN release

This is the first version of MetacodeR on CRAN. Highlights include:

• Functions to parse any text-based file format with taxonomic information.
• A new class called taxmap that holds tabular "observations" assigned to hierarchical "taxa". This class also supports "virtual" columns.
• A set of functions inspired by dplyr to manipulate taxmap objects
• A function to plot heat trees: a taxonomic tree with the size and color of most components mapped to aribtrary variables. Easily makes publication-quality graphs from taxmap objects.
• A function to run in silico PCR from R