NetCatch enables the user to scan a list of labelled urls and select a subset to read
into a file.

class ExtractUrls scans a file in http format and builds a dictionary of urls

class NetCatch is a decorator for a dictionary of links. Each link is indexed
by its label.  Allows the user to select links of interest and read each
selection into its own file. The filename is contructed by appending the label
with an extension of html.

Files can be selected by index, range or label. The destination directory
defaults to the current directory.  The user can specify another dictionary
by passing a list of path segments to the constructor.

    net_catch = NetCatch()
    net_catch = NetCatch( [ 'amylase', 'species' ] )
    net_catch.get_all_urls()
    net_catch.get_urls_by_label( [ 'pig', 'dog', 'cow' ] )
    net_catch.get_urls_by_index( [ 1, 4, 6, 9 ] )
    net_catch.get_urls_by_range( 2, 5 )

This example uses ExtractUrls to build a dictionary of urls.  Then it imports
the links into a NetCatch instance.

    fh = open( file )
    urls = ExtractUrls()
    url_dict = urls.extract_urls( fh )
    net_catch = NetCatch()
    net_catch.import_dict( url_dict )
    net_catch.get_all_urls()