This commits adds Partial Content Delivery, as asked in issue #93. This
enables the "Accept-Ranges" header on all files delivered via
static. It is possible to ask only a certain portion of bytes like
explicitely described in RFC 7233. Of the 3 ways to ask for a range of
bytes (byte x to byte y, everything from byte x to end of file, last y
bytes from end of file), every one of them is implemented, but if a
request is to ask multi ranges in a same request, this implementation
will only account the first range.

Also fixed a edge case bug where a FromTo range ending out of bounds was not treated as a From range (See https://tools.ietf.org/html/rfc7233, section 2.1).