optimize: fix potential null pointer dereference found by coverity (#9)

568    dd("array var map: array size: %d", (int) array->nelts);
569
   CID 258181 (#1 of 1): Dereference null return value (NULL_RETURNS)7. dereference: Dereferencing array_it, which is known to be NULL.
570    array_it->not_found = 0;
571    array_it->valid = 1;

various coding style fixes.

doc: use the markdown format instead; also added more detailed docume…

…ntation.

fixed compiler warnings when DDEBUG is enabled. we also fixed another…

… Visual C++ compiler warning. thanks Edwin Cleton for the report.

now we check the ngx_devel_kit dependency at config time and provide …

…a more friendly error message if it is missing.

fixed a nasty use of unitialized value bug in array_map: we should al…

…ways clear the memory of the ngx_http_compile_complex_value_t object before feeding it into ngx_http_compile_complex_value.

no longer refer to my fork of NDK because my fork has already been me…

…rged back to the mainstream :D