Remove implements Serializable

Sub-class Entiti implements Serializable
Remove redundant "implements Serializable"

Merge branch 'develop'
Refactor getId() -> id()

BeanTableModel uses reflection API.
So, Id is shown in Table due to getId()

Fixes #60
OPEN - task 60: Id Field from method getId() show in Entity Table 
http://github.com/DevOpsDistilled/OpERP/issues/issue/60
Remove Id from EntityTable (Fix #60)

Merge branch 'bug60-remove-id-in-list'
Merge pull request #62 from TheKojuEffect/master

Remove Id column in EntityTable (Fix #60)