v0.6.26

Implemented in this build:

• exception handling with try/with/finally
• use! keyword in actions
• utility functions FailWhen and WhenError related to exceptions handling
• improved for/while/use in actions
• new option DbFileName to define build database file name other than .xake

Example of catching the errors in rules

        ("bin/FSharp.Core.dll") *> fun outfile ->
            WhenError ignore <| action {
                do! copyFile "packages/FSharp.Core/lib/net40/FSharp.Core.dll" outfile.FullName
                do! copyFiles ["packages/FSharp.Core/lib/net40/FSharp.Core.*data"] "bin"
            }