Updated erubis content processor to be compatible to ERB

This should make using erubis instead of erb as easy as
associating the content processor name erb with erubis.
Fixed NodeFinder descendants filter to take already filtered nodes into account
Small performance enhancement by caching normalized link definitions for kramdown
Some performance enhancements and API clarifications for Webgen::Path

The use of the stdlib uri library brings some performance penalties.
These penalties have been mitigated by

* caching the result of Webgen::Path.url so that fewer over-all
  operations need to be done
* by optimizing Webgen::Path.url to avoid unnecessary
  URI::Generic#+ operations

Some other small performance improvements have been implemented as
well throughout the whole class.
Small performance improvements for node finder filters descendants and siblings
Using new Webgen::Path.absolute? method in tag relocatable and path handler virtual
Updated methods in Webgen::Node to not explicitly use self
Optimized Webgen::Context so that accessing various nodes is faster
Name of path handler instance in some error messages was not shown correctly
Removed test for maruku (it is deprecated)
Update version number