expand url regex to allow for userinfo

[bdehamer]

Closes #745

Expands the URL regex used in validate_download_location to allow for a userinfo specifier to appear before the hostname. Previously, a value like git+ssh://git@git.myproject.org/MyProject.git would fail validation due to the inclusion of the git@ portion of the URL.

In the URI spec (RFC2396), the userinfo portion of the server component is defined as follow:

userinfo      = *( unreserved | escaped | ";" | ":" | "&" | "=" | "+" | "$" | "," )
unreserved    = alphanum | mark
alphanum      = alpha | digit
alpha         = lowalpha | upalpha

lowalpha      = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
                "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
                "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"
upalpha       = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
                "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
                "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"
digit         = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                "8" | "9"
mark          = "-" | "_" | "." | "!" | "~" | "*" | "'" |
                "(" | ")"
escaped       = "%" hex hex
hex           = digit | "A" | "B" | "C" | "D" | "E" | "F" |
                        "a" | "b" | "c" | "d" | "e" | "f"

I translated this into the following regex:

[\w\-.!~*'()%;:&=+$,]+

and then placed it as an optional prefix to the hostname (with a trailing @ character)

([\w\-.!~*'()%;:&=+$,]+@)?

[armintaenzertng]

LGTM, thanks for the addition! :)