Added shot() and sequence() funcs

Added functions for returning shot and sequence entities by name.
Removed hard coded entity id
SgProject.sequence() fixed search filter

Search filter changed from "name_is" to "is".
Updated return data for SgFieldUrl local storage

When a SgFieldUrl has a link type of "local" the returned value
now includes the Attachment Entity "id" and other field data
such as "local_path" etc.
SgClassFactory now adds var pointing to base class for Entity

SgClassFactory when building the classes for Entity types now
stores the class used as the base class as a variable
"__sg_base_class__".
SgVersion Entity fixed arg order bug

Fixed a bug where args were being passed out of order to build
the version number.
SgEngitySchemaInfo now loads unsupported fields from XML caches

Unsupported fields written to a schema cache XML file are now also
loaded when reading a cache XML.
SgFieldEntityMulti.value() returning None fix

SgFieldEntityMulti.value() returns None when it should return an
empty list.  Example: A sequence that contains no shots the "shots"
field should return an empty list not None.
SgProject.shotNames ignore sgSequences arg

SgProject.shotNames now passes sgSequences to SgProject.shots.
SgFieldImage.openInBrowser prevent trying to open empty fields

If the url for the field is None or an empty string the field no
longer trys to open the image in a browser.
SgFieldImage.uploadFilmstripThumbnail possible error if parent is None

Swaped to if statments due to parent possibly being None.
SgConnection added baseEntityClasses function and init arg

Its not easier to specify custom Entity base classes for connections
by passing an arg to the constructor and for SgConnection
subclasses defining SgConnection.baseEntityClasses() returning a
dict.

The constructor arg and return result of baseEntityClasses are
merged with the constructor arg overriding any values of
baseEntityClasses.
SgConnection set constructor arg baseEntityClasses default value
SgConnection.projects add sgStatus arg and rename projectTypes arg

Added the ability to filter projects by status and renamed arg
projectTypes to sgProjectTypes.
SgConnection added project, user, users functions

Added functions for returning a project/user by name and also
all users.
Schema and SgFieldSelectionList updates

Schemas no longer save out empty field data, in particular,
valid values, value types,summary info,  data.

SgFieldSchemaInfo added displayValues() function.  Used by selection
list fields, see SgFieldSelectionList commit info.

SgFieldSelectionList added displayValue() for returning the display value.

Status fields store there value as a short name, you can use displayValue()
for returning the user friendly long name.
SgFieldImage monitors expire time of url and is dirty afterwards

SgFieldImage checks the expire time of the url from shotgun and
isValid() will return False if the current time is greater than
or equal to the expiration time thus forcing a refresh from
shotgun for the fields value.
Merge branch 'develop'