as.term method for difftime objects
forwardrate implemented and code organized

- forwardrate.spotratecurve implemented
  - handles numeric and date terms
  - from and to arguments accept vectors, forward argument does not.
- forwardrate.spotrate implemented (must be reviewed)
- indentation changed to 2 spaces, instead of tabs (4)
Code indented with 2 spaces
Code indented with 2 space