optimize: fix potential null pointer dereference found by Coverity (#108

)

245            dd("found opcode echo location async...");
   CID 258040 (#8 of 8): Explicit null dereferenced (FORWARD_NULL)9. var_deref_model: Passing null pointer computed_args to ngx_http_echo_exec_echo_location_async, which dereferences it. [show details]
246            rc = ngx_http_echo_exec_echo_location_async(r, ctx,
247                                                        computed_args);
248            break;

bugfix: config: we should always use double quotes in [...] condition…

…als.

doc: bumped version to 0.61.

feature: nginx 1.11.11+ can now build with this module.

Note: nginx 1.11.11+ are still not an officially supported target yet.
More work needed.

Closes #64

See also:
http://hg.nginx.org/nginx/rev/e662cbf1b932

Signed-off-by: Yichun Zhang (agentzh) <agentzh@gmail.com>

doc: make it clearer when to use the -- form.

doc: version 0.59 and nginx compatibility as far as 1.9.15.

feature: this module can now be compiled as a dynamic module with NGI…

…NX 1.9.11+ via the --with-dynamic-module=PATH option of ./configure.

style: minor fixes.

bumped version to 0.57.