small fixes

surface_downwelling_shortwave_flux should
surface_downwelling_shortwave_flux_in_air according to wiki
simple met workflow
use units from ameriflux file for time
fixes to gapfill

simplify posix date based on info in nc file
surface_downwelling_*_flux should have _in_air at the end
all functions now take start_date and end_date

Dates are assumed to be PosixT objects. All functions to do download
and conversions will work on full years. Only function that should work
on date range is the met2model functions.
Merge pull request #283 from PecanProject/fix-metworkflow-2

Fix metworkflow 2
updated trait data and density plots to use updated ggplot2 syntax. closes gh-93
updated DESCRIPTION and NAMESPACE for PEcAn.priors
Merge branch 'master' of github.com:PecanProject/pecan
updated documentation for PEcAn.priors
commented out plot.prior.density example
fix build