Added the hash attribute to ints and longs.

FlatBuffer schema files can now optionally specify a hash attribute that
will allow someone writing json files to enter a string to be hashed
rather than a specific value. The hashing algorithm to use is specified
by the schema.

Currently the only algorithms are fnv1 and fnv1a. There are 32 bit and
64 variatns for each. Additionally, a hashing command line tool was
added so that you can see what a string will hash to without needing to
inspect the flatbuffer binary blob.

Change-Id: I0cb359d0e2dc7d2dc1874b446dc19a17cc77109d