Ensure correct order of variable names when reading ascii files

pandas.read_table() does not preserve the column order in usecols=. For this reason, the column names (which are associated with specific netcdf variables) need to be sorted in increasing column order before pandas.read_table() is used.
Merge remote-tracking branch 'upstream/master'
Merge pull request #6 from bartnijssen/master

Ensure correct column order in reading VIC ascii files