doc: documented that the coroutine.* API can be used in init_by_lua since 0.9.2. thanks Ruoshan Huang for the reminder.
reduce compiling warning(avoid halting compiling)
Merge pull request #322 from zh-ang/master

fixed a compilation warning when PCRE is disabled.
change: ngx.decode_base64() now only accepts string arguments.
refactor: avoided the magic error code -3 in ngx_http_lua_ffi_get_ctx_ref.
ngx_http_lua_ffi_set_ctx_ref: return the error code NGX_HTTP_LUA_FFI_NO_REQ_CTX properly.
added (passing) tests for feeding lua numbers to ngx.md5(), ngx.md5_bin(), and ngx.sha1_bin().
added a (passing) test for providing the resulting match table to ngx.re.match().
feature: added the optional 5th argument to ngx.re.match() which is the user-supplied result table for the captures.

This feature gives 12%+ speedup for simple ngx.re.match() calls with 4
submatch captures.
feature: added new pure C API, ngx_http_lua_ffi_max_regex_cache_size, for FFI-based implementations like lua-resty-core.
added (passing) tests for using lua numbers in response header names and values for the ngx.header.HEADER API.
shdict: improved the error handling a bit and also increased the tests' code coverage.
feature: ngx.escape_uri() and ngx.unescape_uri() now accept a nil argument, which is equivalent to an empty string.
ngx.var: improved the error handling a bit.
ngx.var: added a (passing) test for writing to a non-changeable nginx variable.
shdict: improved the error handling a bit and also added more (passing) tests.
reindexed the test file 043-shdict.t.
added a (passing) test for error status to improve code coverage.
ngx.re: added the whole regex flags string to the error message when there is an unknown regex flag.
bugfix: segfault might happen when aborting a "light thread" pending on downstream cosocket writes. thanks Aviram Cohen for the report.