ledger-mode: define and use a syntax table so that comments are handled properly

Specifically, functionality which uses syntax-ppss to detect comments
needs the syntax table to be aware of them. A prime example is
goto-address-prog-mode.

This commit adds a basic syntax table for this purpose.
Merge pull request #220 from purcell/ledger-mode-syntax-table

ledger-mode: define and use a syntax table so that comments are handled properly