[RELEASE] Released version 1.3.15.3

[RELEASE] Released version 1.3.15.3

Released version 1.3.15.3 with the following main changes :
    - [BUG] disable buffer read timeout when reading stats
    - [BUILD] change declaration of base64tab to fix build with Intel C++
    - [BUILD] silent a warning in unlikely() with gcc 4.x
    - [BUG] use_backend would not correctly consider "unless"
    - [CLEANUP] remove dependency on obsolete INTBITS macro
    - [BUG] fix segfault with url_param + check_post
    - [BUG] server timeout was not considered in some circumstances
    - [BUG] ev_sepoll: closed file descriptors could persist in the spec list
    - [BUG] maintain_proxies must not disable backends
    - [BUG] regparm is broken on gcc < 3
    - [OPTIM] force inlining of large functions with gcc >= 3