add user defined event map
This should fix #1016.

Usage for #1016:
Create a file named "custom.event.map.lua" in the koreader directory
with the following content:

return {
    [102] = "LPgFwd",
}
load Device module after DEBUG is turned on
otherwise some DEBUGs are not printed.
Merge pull request #1091 from chrox/fix_1016

add user defined event map