Add default.nix file, for building with Nix
The option --permissive now quiets balance assertions
Add the concept of "deferred postings"

This is pretty much exclusively for allowing one to use balance
assertions with replicated transactions across multiple files.
Update tools/build.sh
Bump the date in the version number
Apply any outstanding deferred postings at the end of parsing
Correct one of the baseline tests
Add a safety check when using deferred postings
Improve an error message relating to balance assertions
Add ninja as a build dependency
Replace sha1.cc with boost::uuid::details::sha1

sha1.cc is not redistributable by Debian because the license doesn't
permit redistribution of modified versions.

This isn't ideal since the ::details namespace is subject to change,
but it avoids adding a dependency to ledger.
Require the use of C++11
Temporarily disable the DocTests
Bump minimum Boost dependency to 1.55
Have Travis download Boost-1.55
More Travis changes
Another .travis.yml fix
Merge commit 'a0c5addbbdaf472d26ae25c86ff826e509b3ee17'