Add support for memcached monitoring about appmetrics

[seabaylea]

I've prototyped support in my personal fork (https://github.com/seabaylea/appmetrics#memcached). This produces the following data:

Event: 'memcached'

Emitted when a data is stored to or retrieved from Memcached using the memcached module.

• data (Object) the data from the memcached event:
  • time (Number) the milliseconds when the memcached event occurred. This can be converted to a Date using new Date(data.time)
  • method (String) whether the event was for a data set or get.
  • key (String) the key associated with the data.
  • duration (Number) the time taken for the get or set of the memcached data to occur.

This is also included in request tracking, but neither a set or a get will start a 'request'.

Any feedback anyone has is appreciated.

[seabaylea]

The prototype covers calls to the following functions:
get, gets, getMulti, set, replace, add, cas, append, prepend, increment, decrement, incr, decr, del, delete

I've not added support for version, flush, samples, slabs and items as they seem to be more one-off/non-performance critical calls.

This can easily be added if there's a requirement though.

[seabaylea]

Closing based on PR #94