Add a build status image from Travis-CI.

This will only work if @ccgus adds fmdb to Travis-CI.
Merge branch 'testcases' into develop
FMWALDatabaseCoordinator: a reader-writer queue.

FMWALDatabaseCoordinator only works for SQLite databases with the Write-Ahead Logging (WAL) journal mode.  This journal mode permits concurrent readers with serialised writes.  So readers will never block a writer and a writer will never block a reader.  But you can only have one writer at a time.

Read operations should be sent to `inSnapshot:` which performs read operations in a "DEFERRED" transaction, guaranteeing a consistent view of the database even in the presence of concurrent writes.

Write operations can be sent to the FMDatabaseQueue 'writeQueue'.  There is also a FMDatabasePool 'readPool' which is used for 'inSnapshot', but 'inSnapshot' should be preferred as it creates 'deferred' transactions instead of the 'exclusive' transactions created by -[FMDatabasePool inTransaction:] which can block writers.

Creating a FMWALDatabaseCoordinator will try to change the journal mode of the database to 'WAL', and will fail if the journal mode can't be changed.