Changed handling of blank environment, fixed section handling on command line, cleaned up testing, added release changes

* Changed behavior if environment name is set but empty: now uses default instead of looking for '' environment
* Fixed command line handling: different variables in the same section are now all recognized (see IssueID #5501). The test cases are updated accordingly
* Re-organized test cases to group command line handling in its own class
* Added empty [namelists] section to test config, simplifying test writing
* Added CHANGES.txt for upcoming release 0.3.0, updated setup to 0.3.0dev

* Added traversal (walk) function to parse keys and values for verbatim target variables, i.e. variables of the generated script language
* Added environment functions to temporarily reset the string used for new lines. Needed to work around the missing 'wrapstring' keyword in Jinja 2.6
* Added context function to allow querying context variables by name within templates. Needed to get default values for target variables.

Added configurable variable format to namelist values. Variables $${x} are now recognized as script native variables. Defining .var_format allows different script syntax, e.g. XML style (.var_format = <%s>)

Fixed override behaviour for global job config: now specialized job config will always override global job config

* Added new control variable 'EXP_OPTIONS', taking a list of config names. Options are taken from the new 'standard_options' subdirectory
* Added respective test and simplified test module

* Added setting of sectioned variables via command line, using '.' as section path separator and '..' for verbatim periods
* Added automated testing using Python's unittest library
* Disambiguated warning due to missing experiment configuration; now mentions 'experiment'.