Travis and Coveralls support

Builds `pg_shard` against PostgreSQL 9.4 in Travis and uploads coverage information to Coveralls. Certain tests had problems that were exposed in the Travis environment, which have been corrected.

Additionally, the scripts in the Travis gist have been modified to build a cluster directly owned by the Travis user. This keeps us from needing to run a bunch of chmod/chown commands before tests.

Resolves #24.