Add possibility to add custom properties for code elements

Introduce a base interface to ClassInfo, MethodInfo, PackageInfo, FileInfo, StatementInfo, BranchInfo which allows API clients to add arbitrary properties to the model. They values should be raw types, possible Serializable objects too. This should allow instrumentation providers better answer questions Clover may have about the model they've produced.

Something like:

interface CustomInfo

• getProperties(): Map<String, Object>
• setProperty(String, Object)
• getProperty(String)

interface FileInfo extends ..., CustomInfo
interface TypeInfo extends ..., CustomInfo
interface MethodInfo extends ..., CustomInfo
interface BranchInfo extends ..., CustomInfo
interface StatementInfo extends ..., CustomInfo