Added Ascii85HttpHeader class and tests

Ascii85 is very efficient in converting binary to printable ASCII characters. Unfortunately, if the application is to use those printable ASCII characters in an element of an HTTP header, then the string is not allowed to contain separator characters like {()<>@,;:\"/[]?=}, most of which are used in Ascii85-encoded strings. The Ascii85HttpHeader class contains an additional encoder/decoder pair that replaces disallowed separator characters in an Ascii85 string with allowed characters. If you think this might be useful in general, here it is. If some of those characters are allowed in a particular application, but not others, it is easy enough to adapt this code to suit.