Added Guava 14.0.1 as test dependency to reduce the amount of
boilerplate code when writing test cases
Added test to reproduce the writing of wrong content into the cache file
while a remote host answers unexpected data (belongs to issue #23)
Changed method signature to read and save 'UAS data' into a cache file

(preparations to fix the error of issue #23)
Added additional method to data readers to read 'UAS data' directly from
a given string (belongs to issue #23)
Added check to verify the readability/validity of data store's content
before writing them into a cache file (fix for issue #23)
Fix to read in string-based data sources with default charset (UTF-8)