tools: make clean avoid sandboxes
make build warnings consistent
tools: more selective make clean*
tools: make uninstall, reduces noise when testing

Tries to unregister all three hledger packages from the user's package
db (not cabal sandbox aware).
tools: document ghci*, repl* rules
fix a warning in TransactionsReports
web: clean up warnings
clarify similarAmountsOp doc
tools: make allcabal* -> make cabal*
switch to Decimal for representing quantities (closes #118)

hledger has represented quantities with floating point (Double) until
now.  While this has been working fine in practice, the time has come to
upgrade our number representation to something more principled: Decimal,
for now. As a bonus, this brings a ~30% speed boost to most reports.

We'll keep the old representation(s) around for a while, selectable via
hledger-lib cabal flag, for research/testing/benchmarking purposes. To
build with the old Double representation: cabal install -fdouble
hledger-lib hledger hledger-web
cli: add a hidden --version+ flag showing number representation (#118)
cli: --version shows "dev build" when VERSION is undefined

Probably only noticeable in GHCI.