SgField __init__ add arg for parent entity, minor cleanup for fields

SgField __init__ now accepts an arg for the parent entity, also did
some cleanup for "id" and "type" fields setting the var of _valid
instead calling super.setValid().
Fixed incorrect var name reference in SgField.__init__
Added isUserField to SgFieldSchemaInfo

Added isUserField to field schema info class and remove SgField
function checking its info class for deteremining if is a user
field instead hit the info object.

Cleanup of field functions calling schemaInfo() instead of direct
var.
SgFieldImage added isLinkExpired() function

Added function for testing if the image link timeout has expired.
Warn when exporting schema that was built from a cache file
Added support for "timecode" fields

The return type "timecode" is now supported as a field.

If the value in shotgun is greater than the number of milliseconds
in a day the field will push a warn msg to the SgField logger.

SgFieldTimeCode can not have its value set to a value greater
than the milliseconds in a day by calling SgField.setValue()
SgSchema fixed deadlock issue within initialization process

SgSchema._initialize now triggers the thread wait event before locking
itself down.
Added the ability to selectivly cache/not cache an Entity

It is now possible to specificaly tell an Entity whether or not
to cache.

By default Entities determine if they should cache themselves based
on there SgConnections caching state, this can be overridden by
calling the SgEntity.enableCaching() function.  To disable caching
for a specific Entity the SgEntity.disableCaching() func is avail.

To reset an Entity to use the connections caching state there is
the SgEntity.resetCaching() function.

When an entity falls out of scope its caching state is remembered
should it be created again.
Doc cleanup
Added classes for monitoring EventLogEntry

The event log classes are structored alot like pythons classes in
the logging module

SgEventFilter - logging.Filter
SgEventFilterer - logging.Filterer
SgEventHandler - logging.Handler
SgEvetnWatcher - logging.Logger
Added searchFields() func to example user field SearchExpressionField

Search expression field can now define fields to fill the returned
results.
Added SgSequence to example Entities.
Merge branch 'develop'