Reimplement as a single multigraph plugin

[rectalogic]

This is a pretty big change, it consolidates all plugins into a single mongo_munin multigraph plugin.

It is compatible with mongdb 2.4+ and 3.x. The lock output is very different between the two, and the btree graph is replaced by page_faults which is what mongo recommends (since btree graph is not supported in 3.0)

It also adds a graph of all collections - their document count and byte size.

If you decide to merge this PR, you can ignore my other PRs - they are incorporated in this one.