Add last_update and starred to CompoundIndicators.

Last_update represents the last timestamp in which the indicator data
was updated. Starred represents if the indicator must be shown in the
front page.
Closes #29
Deleted latest change, troublesome and no longer needed, solved issue #29
on API
Add MonthInterval entity. Close #30

Add MonthInterval entity, representing a single month.
Add 'value' field to all the Interval entities.
Fixed NoneType error on interval constructor
Fix #32

MeasurementUnit.name length was increased from 20 to 100 characters
Merge branch 'bugfix-#32' into staged-changes
Close #31
Merge branch 'enhancement-#31' into staged-changes
Fix #33
Merge branch 'bugfix-#33' into staged-changes
Close #34

Add 'convertible_to' and 'factor' to MeasurementUnit.
'convertible_to' represents the normalized measurement unit name
and 'factor' contains the product factor to make the conversion.
Merge branch 'enhancement-#34' into staged-changes
Closes #35
Merge branch 'enhancement-#35' into staged-changes
Increase Topic.id length

We need to increase the Topic ID length in order to use more
descriptive IDs, such as LAND_TENURE instead of TOP1
Close #36

MeasurementUnit.factor has been changed to a Float, since it can contain
values such as 0.001 or 1000.0.
Merge branch 'enhancement-#36' into staged-changes
Add latest changes
Merge branch 'staged-changes' of github.com:weso/landportal_model into staged-changes