parse with length option

[douxing]

I think that GenericDocument::Parse should have a version with length option, like:

GenericDocument::Parse(const Ch* str, size_t length)

it might be redundant but is very handy with communication data, thus i don't have to reallocate memory for the \0 at the tail after receiving the data.

[gggin]

you can use std::string version

[pah]

This is available in the current master since the merge of #553, see also #158 for more discussion.

[miloyip]

Thanks @pah

[douxing]

@pah @miloyip after a git pull, every thing is okay, thank you!