define get_alter_maql for every column
define maql templates to alter date, label and hyperlink
DictDiffer does not need to return new and old attributes, just new
factorize AlterColumn class
migrationchain.simplify function : slice complex actions in simpler ones
make it possible to query existence of column with title argument
in column.get_alter_maql: split date between date and time, this was forgotten
make it possible to pass references_cp argument at column creation. Useful for Labels
bugfixes on Label.get_alter_maql: the template needs to be != to empty for changes like HyperLink->Label
typo fixes in label an hyperlink ALTER_TITLE templates
some fixes in labels / hyperlinks actions: references_cp support and label<-->hyperlink support
add support for complex migrations (splits) on Date columns
add exhaustive tests to comlex migrations and check them
typofix in AlterColumn comment
Merge branch 'master' into mvdb/ANA-460/complex-migrations

Conflicts:
	gooddataclient/columns.py
Merge branch 'master' into mvdb/ANA-460/complex-migrations
Merge branch 'master' into mvdb/ANA-460/complex-migrations
some typos fix before review
Merge branch 'master' into mvdb/ANA-460/complex-migrations
added `fact` parameter to dataset.has_column