add import handling for namespaces

Added import statement (the use ... lines) extraction to the GetCurrentNameSpace() function
Added import aware class/namespace expansion to ExpandClassname
Added import awareness to Classname and General completion
Added import awareness to GetClassContent()
Related to #4
Make class extracion from source more robust
Enhance use ... completion

Removed leading slash because PHP manual doesn't recommend those
Don't add built in class names when there's already a namespace part typed in
Make "(new Class)->" lines be picked up by the correct completion branch

Previously, while GetClassName() would recognize this pattern, the
code always threadted this context as a class name completion because
of the new keyword.