Monitoring of mBean attributes

[soenkekueper]

We are currently evaluating on how to use JavaMelody within our applications.
As we use Oracle BerkeleyDB JE as embedded Database we want to monitor some of the database-internal performance values and some application-related parameters. Both of them are already provided within as mBean values. So we need an easy way to include this mBean Attributes into JavaMelody Monitoring.

With this pull request i've added an Parameter mbean-values that allows the selection of a mBean Attribute list, that will be included within monitoring.
The parameter value must have the following format:

AttributeSelection[|AttributeSelection]*
AttributeSelection:= <Display Name>#<ObjectName>#<AttributeName>

This may look like this:
Active transactions#CustomBean:type=DatabaseEnvironment#TransactionCount|Cache misses#CustomBean:type=DatabaseEnvironment#CacheMisses

When specified the following mBean values will be monitored as additional values and displayed in the extended counter section:

Display name	ObjectName	mBean Attribute
Active transactions	CustomBean:type=DatabaseEnvironment	TransactionCount
Cache misses	CustomBean:type=DatabaseEnvironment	CacheMisses

If an attribute is selected that doesn't exist or if the Bean doesn't exist, an warning will be logged an the attribute will be ignored.

The result may look like this: (Lange Transaktionen, Kurze Transaktionen, Alter des Backups, Min. Dateiauslastung, Max. Dateiauslastung, Freier Speicher, are values collected from mBeans)