Add Rais image server

Closes #1 (closed)

Versions 4.0.1 and 4.0.2 cause an issue with large JPEG files.

With no concurrency and 4 iterations I got those results (JPEG):

server	requests per second
cantaloupe	6.35
loris	7.22
rais	3.96

Degraded performances are mostly observed retrieving full images (4x slower than loris for any file size). It has a higher std deviation (44ms vs <14ms for loris/cantaloupe)