Port to the new HLedger lib that's based on Decimal numbers instead of Double.
Minor cosmetic updates.
hledger-interest.cabal: tentatively bump version number for next release
Hledger/Interest/Rate.hs: implemented a simple parser that can read an interest rate table from a text file
Dropped --constant interest rate.

The "constant" interest rate was a bad idea that never worked.
bgb288.irt: interest rate table
README.md: modern hledger versions handle the "1 hour @ EUR 35" syntax just fine
Read interest rate tables from file, instead of hard-coding them into the binary.
Hledger/Interest/Rate.hs: avoid compiler warnings
Hledger/Interest/Rate.hs: fixed table2rate function

Currently, this function uses a magic date 999999-12-31 to denote the day when
the last rate in the table becomes invalid. This is a bad idea. It would
probably be wiser to enforce that the user ends the IRT file with a pseudo
entry that conveys this kind of information.
Fixed the table2rate function, at last.

If a lookup goes beyond the last entry of the table, we fall back to
"annual" interest using the rate of the last known entry.

Furthermore, there are various style improvements, which were suggested
by hlint.
Updated build for hledger-lib version 0.18.
release.nix: add build instructions for continuous integration with Hydra
release.nix: delete obsolete file

The build instructions for Hydra reside in the "ci" repository.
bgb288.irt: add official interest rates up until 2014