chainable constructions and Time-to-string conversion

[rompetroll]

This change introduces two improvements for outgoing mappings (reading from layer):

1. support for using time.Time item values in places that require string. e.g. UriPatterns, various constructions like concat, replace etc. The mapper will now autoconvert time.Time values to ISO8601 string format for this purpose
2. ability to chain multiple constructions together for more options when composing values. See included unit test for possible usage benefits