Easyconfig stringtemplate

allow templating and templating constants in the easyconfigs eg
```python
sources = [SOURCE_TAR_GZ]
source_urls = [(GOOGLECODE_SOURCE)]
installopts = 'PREFIX=%(installdir)s'
```
installdir is the installdir which you can't access in any way from the easyconfig 
SOURCE_TAR_GZ is the typical "%(name)s-%(version)s.tar.gz" etc

there is also documenation what can be done/used through
```python
generate_template_values_doc()
```