travis-ci: enable continuous integration on http://travis-ci.org/
git-annex.cabal: relax 'mtl >= 2.1' constraint to 'mtl >= 2'

Git-annex doesn't actually depend on mtl version 2.1. By relaxing this
constraint, builds can succeed on Ubuntu 12.04.
git-annex.cabal: declare dependency on <sys/event.h> header file when building on BSD
git-annex.cabal: let the 'Fast' flag default to 'False' unless it's explicitly specified

The current use of the 'Fast' and 'Production' flags is both inconsistent and
redundant. It's inconsistent, because users are allowed to specify both '-fFast
-fProduction' at the same time -- thereby enabling two contradicting features.

The flags are redundant, because Cabal allows users to specify '-O0' at
configure time, which is essentially the same as '-fFast'. Since 'Production'
is just the opposite of 'Fast', this means that neither flag is needed.
Makefile: don't rely on the cabal-install executable to run the build
Build/mdwn2man: don't rely on perl being installed in /usr/bin; instead find it in $PATH