Added options for what to do when the hook response exceeds 65K

As I stated in my comment to the parent changeset, I believe that returning true when the output exceeds 65K is a bug (and it could lead to change sets which violate what the hooks are designed to keep out).  

If you think that I am wrong about this, I have also coded a [change](https://github.com/rappazzo/atlassian-external-hooks/commit/9214e79af9787f93329da72ec21bbaf558a0c116) that would allow the hook setup to specify what occurs when there is excessive output.  In that case, please reject this pull request, and indicate that I should make a pull request for that change set.