Address #233: support blob_value in protobuf

[tseaver]

The only sane way to do this is to map "text" onto 'string_value', and "bytes" onto 'blob_value'.

Since native 'str' is really "bytes" under Python2, users will need to get things decoded before setting them onto their entities if they want them to be saved as 'string_value'.

[coveralls]

Coverage remained the same when pulling 6f50604 on tseaver:233-support_blob_value into 86b308a on GoogleCloudPlatform:master.

[dhermes]

I think this will confound people but also agree this is the most sane way to go.

In a future where we have ndb-like features, the distinction will be clear.

LGTM

[tseaver]

Rebased to deal with merge conficts.

[coveralls]

Coverage remained the same when pulling d5642c9 on tseaver:233-support_blob_value into 20ac9ef on GoogleCloudPlatform:master.

[tseaver]

Do we need to open a new issue about documenting how people can ensure that the values they set onto entities get mapped as they expect? It is (eventually) going to have to educate folks that 'str' is "bytes" on Python2, but "text" on Python3, and mandate that they be explicit.

[dhermes]

Yes I like that idea. Still LGTM if you want to merge.

[tseaver]

#266 carries the water on the documentation issue.