lwan-request.c: Fix integer conversion warnings
lwan-response.c: Fix integer conversion warnings
lwan-serve-files.c: Fix integer conversion warnings
lwan-status.c: Fix integer conversion warnings
lwan-template.c: Fix integer conversion warnings
lwan-thread.c: Fix integer conversion warnings
lwan-trie: Fix integer conversion warnings
realpathat.c: Fix integer conversion warnings
strbuf.c: Fix integer conversion warnings
base64.c: Fix integer conversion warnings
main.c: Fix integer conversion warnings
lwan-io-wrappers.c, lwan-socket.c: Fix integer conversion warnings
murmur3.c: Fix integer conversion warnings
Add '-Wconversion -std=gnu99' to C flags
Use '\x25' instead of '\%' in error response template

Closes #40.