fix to adapt to changes in data.table::fread

Uploaded a fix to adapt to the functions broken as a result from changes in fread.

linux choose_dir fixed

fixed problem with returned value from zenity

fread_zip added

added fread_zip to operate the same as loadcsfromZIP only way faster,
also added the prefix option for fread_folder and fread_zip

choose_dir special

Added choose_dir for Mac OSX and Linux users,
creates a prompt window if you want to select a folder interactively.
also a function for identifying the OS and an implementation of choose_dir in loadcsv_multi and fread_folder functions.

updates and changes

'txt' argument changed to extension in fread_folder and loadcsv_multi.

README and NEWS updated.
updated version to 1.0.3.
zip and URL still use 'txt' parameter.