Support for magnet uris.

Added support for handling magnet URI when adding using -u option.

Examplary usage:
btc add -u
"magnet:?xt=urn:sha1:6de6a83d402817ef5116d28072f9dd6b0600c6d6&dn=OOo_2.1.0_LinuxIntel_install_en-US.tar.gz&xs=http://mirror.switch.ch/ftp/mirror/OpenOffice/stable/2.1.0/OOo_2.1.0_LinuxIntel_install_en-US.tar.gz"