googleapiclient.http: guard when importing ssl

Fix #191 by adding a guard when importing `ssl`, and replace all direct
references to `SSLError` (the sole member of `ssl` being used) with a
shim, `_ssl_SSLError`. It is prefixed with a '_' to caution users
against relying on the name when importing `googleclient.http`.

`httplib2` also takes a similar approach (see `ssl_SSLError` in
`httplib2/__init__.py`).