Using StringBuilder to build Json and Xml, updated .gitignore and other minor changes

Using StringBuilder to concatenate strings, part II.

Changed toXml method to `toXml(boolean pretty, boolean declaration, String ... attrs)`, symmetric to the existing `toJson(boolean pretty, String... attrs)`. Corrected indentation for both methods. 

In toJson, model_class attribute was removed at commit 5d65fcc05a3d277561209c40ed77dbb44f0804c8. Also, dates are not outputted in ISO format, as SimpleDateFormat apparently does not supports its time zones. Updated the test cases.

Added a few join methods in Util class. I'm using them in my new code. ~~I'm promising to myself to review them -- add documentation and remove unused methods -- after I finish~~ It is documented and with corresponding tests.

Using the cool https://www.gitignore.io .gitignore generated file. Hope you like it!  ;- )