Improve css

[johnd0e]

• unify indents
• refine some values
• smart align any leaflet control (not only zoom control / zoom slider)
• examples: add options (mainly for easy position change)

This PR fixes last left item in https://github.com/kartena/Leaflet.Pancontrol/blob/master/TODO.md:

• Do something about the hardcoded support for the zoom- and zoomslider controls.

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This change is 

[johnd0e]

Alternatively we could use margin instead of position:

diff --git a/src/L.Control.Pan.css b/src/L.Control.Pa
index 071cfbb..b8f7c39 100644
--- a/src/L.Control.Pan.css
+++ b/src/L.Control.Pan.css
@@ -6,10 +6,10 @@
 }

 .leaflet-left .leaflet-control-pan {
-       left: -24.5px;
+       margin-left: -14.5px; /* 24.5-10 */
 }
 .leaflet-right .leaflet-control-pan {
-       right: -24.5px;
+       margin-right: -14.5px;
 }

 .leaflet-control-pan {
@@ -86,10 +86,10 @@
 }

 .leaflet-touch .leaflet-left .leaflet-control-pan {
-       left: -26px;
+       margin-left: -16px; /* 26-10 */
 }
 .leaflet-touch .leaflet-right .leaflet-control-pan {
-       right: -26px;
+       margin-right: -16px;
 }

 .leaflet-touch .leaflet-control-pan {