improve DbFunctions
improved DbFunctions 2
minor: make tx fn invocation consistent
support imports and requires for Db function

clojure code can take imports and requires as part of the map that
makes up the database function
minor: change apply for InvokeTxFunction
add missing apply cases for InvokeTxFunction

and reformat code for better code width
rename special case of transaction functions

some code reformatting to reduce code width
transaction functions are a special case of database functions, as they expect a database value as a first parameter, and this argument will be automatically supplied by the transactor.
optional args to Add(Db/Tx)Function methods

ditch case class for (Typed)AddDbFunction
fix optional param for creating type tx fns

move the optional partition parameter to a later parameter list to avoid overloading problems
Merge pull request #54 from pellucidanalytics/features/DbFunction-BIS

Add support for storing and invoking database functions.