Add -a <action> parameter to JavaSearch and JavaSearchContext

The parameter allows to specify where the search result should be
opened, e.g. vsplit would open the result in a vertical split. If the
action parameter is not specified it defaults to g:EclimJavaSearchSingleResult,
which itself defaults to g:EclimDefaultFileOpenAction.

This parameter works for the JavaSearch and JavaSearchContext commands
and in the case of JavaSearchContext it is the only parameter.

Examples:
Open single search result for 'Foo' in a vertical split:
  JavaSearch -p Foo -a vsplit
Open single search result for 'Foo' in the current window:
  JavaSearch -p Foo -a edit

In my workflow most of the time I open files in a virtual split, so I set the
g:EclimDefaultFileOpenAction to vsplit. But sometimes I just want to bring
another Java class to the front without keeping the old one open, so in that
case I prefer the 'edit' action. So this commit tries to make the switch between
different actions more flexible.

So far the documentation wasn't changed, but if you like this little functionality, 
I would of course add the parameter to the documentation.

p.s. the name 'action' isn't probably the best one, but I couldn't come up with
anything better so far.