initial commit of new configuration access logic

1. Introduced new configuration manager class (singleton) based on apache commons composite configurations to load the config files during initialization
2. New configuration properties and their default values are maintained as enums.
3. qpid configuration classes were moved to "org.wso2.andes.configuration.qpid" from "org.wso2.andes.server.configuration"
4. MQTT configurations moved into andes config file
5. Added "commons-jxpath" since it is required by "commons-configuration" to handle xpath queries.
6. Added meaningful comments to describe properties and their usage.

NOTE : There is a significant namespace refactor here. the qpid configuration classes were moved from "org.wso2.andes.server" package to the "org.wso2.andes.configuration.qpid" package. These sources are not modified by wso2, rather re-used.  

Further Refactoring : 
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This commit was not merged since it had conflicts. However, comments on this and further re factoring has been carried out in following pull requests : 

https://github.com/wso2-dev/andes/pull/170 , https://github.com/wso2-dev/andes/pull/172 , https://github.com/wso2-dev/andes/pull/180 , https://github.com/wso2-dev/andes/pull/185
Yes. This exception will be thrown only if both the value read from configuration file, and the defined default value for that property is also wrong. Since we are responsible for defining the default values at code, this exception will ideally never occur. So handling it in multiple places outside is redundant. 

What happens now is, in case even the default value is invalid, the configuration value will be returned as null. Even though an NPE is returned, still the error with the failed configuration property is logged. (An NPE here anyway means that the configuration definitions in code itself are wrong.)

Refer pull request : https://github.com/wso2-dev/andes/pull/220 for this change.