Static cache of ProfiledMongoServer instances

Borrowing from MongoClient.GetServer() and MongoServer.Create(), use a static dictionary to cache instances of ProfiledMongoServer so identical instances will not be created, leaking server connections.
New method to create ProfiledMongoServer instances
Depricate ProfiledMongoServer constructors, setting for max instances (#83)

The constructor removed by
https://github.com/kalahari/MiniProfiler.NET/commit/8aec81aa294ccf41a1ee0d3068495dfa358acd45
is re-created, and both constructors are marked obsolete.  The hard
limit for ProfiledMongoServer instances is now in a configurable value.
Move MongoDB specific settings to MongoDB project

Created MongoMiniProfiler.Settings for settings specific to the MongDB
project, fixed spelling in Obsolete attributes.
Merge pull request #85 from kalahari/issue83

Issue #83: Static dictionary of ProfiledMongoServer instances