Improved performance of compile_list

Compile list now uses a more reader-friendly version the taxon table
(pollen.equiv.RData, also added, with help page) and returns a full
pollen object, just like the one returned by get_download.  It's also a
bit faster since it doesn't need to pull down data from the API, using
only data stored in the pollen object.
Merge pull request #40 from SimonGoring/master

Improved performance of compile_list