add uri validation to core
rename Nugget.text to Nugget.desc in core
fix TestSqlite.test_create for changed behaviour in __init__
change order of add_nugget sql statement because fields are in alphabetical order
change User.add_nugget to accept nugget values rather than an instantiated Nugget object
Nugget invalid url/description handling:
 * modify error messages
 * add unit tests
 * rearrange tests in testbasic into TestNugget, TestUser and TestTask
web: improve error handling in add_confirm
web: handle duplicate nugget errors
improve testing of sqlite (new tests and fix others)
add test/__init__.py so nosetest can call individual submodules