Feature/subroutines with promises

[callmephilip]

This adds support for async subRoutines through replyAsync as discussed in #77 ⚡

[kirsle]

Add documentation for the callback argument. It looks from the source that it should be a function that accepts (error, reply) as parameters?

Also provide examples of both use cases, like

# Example: using promises
#
# ```javascript
# rs.replyAsync(user, message, this).then(function(error, reply) {
#     console.log("Bot>", reply);
# });
# ```
#
# Example: using the callback
#
# ```javascript
# rs.replyAsync(username, msg, this, function(error, reply) {
#     console.log("Bot>", reply);
# });
# ```

(the documentation uses GitHub flavored markdown)

[kirsle]

Could you add an example to the eg/ folder? Either update/replace the existing async-object example or make a new one. It should exercise all the different use cases that the unit tests do (I wouldn't expect an end user to read the unit tests to find out how to use the module 😉).

If not I can add one sometime later after this PR is merged.

[callmephilip]

Added some documentation, as per your suggestion. working on a small example now

[callmephilip]

I've added an example using replyAsync in eg/reply-async ☔