Implement interface "Browsable" for classes providing page turning

This interface is intended to be used everywhere a Zend_Paginator
is returned within our code.
Add "Fetchable" interface for classes providing data retrieval

Classes for retrieving data must implement the Fetchable interface
which defines the usual fetch*() functions.
Implement "Limitable" interface for retrieving just a portion of a result set

Defines how to set a limit count and offset and should be used everywhere
where setting limit and offset is supported.
Add interface "Sortable" defining how a result set can be sorted

This interface should be used to ensure that sorting a result set
is standardized among the different query implementations.
Add interface "Filterable" (WIP)

This interface is empty yet. It's meant to define how to filter a result set.
Add interface "Queryable" (WIP)

Interface for classes which act as a data source and thus return or are a Fetchable.
The name of the interface is not yet final though.
Let the BaseQuery implement the query interfaces and remove filter functionality

This commit breaks the application as it introduces incompatible changes which
will be adopted bit by bit.

Since the filter functionality is subject to change it's removed from the BaseQuery.
The functions setOrderColumns() and getOrderColumns() are deprecated because they're
replaced by functions in the Sortable interface.
Further the Sortable interface now defines the sort constants ASC and DESC and are
thus removed from the BaseQuery. In addition the sort constants are no longer integers
but strings.
The distinct() and isDistinct() functions are removed because they're database specific.
They have been introduced with changes in the PivotTable implementation which should
actually not call distinct blindly anyway.
Create count and select database queries separately

The previous implementation always created both the count
and the select query.
For readability the property baseQuery has been renamed to select which the
IDO queries must take into account.
Db/Connection: Replace getConnection() with getDpAdapter()

For readability getConnection() is deprecated in favor of getDpAdapter()
since Db/Connection is already the connection.
Replace DatasourceInterface with Selectable
Db: Implement missing fetch*() functions on connection level

The BaseQuery forwards fetch*() function calls to its datasource with the
query as first parameter since the datasource implements the actual fetching functionality.
There's no interface for fetch*() functions on the datasource level yet.
CommentQuery: Rename baseQuery to select
ContactgroupQuery: Rename baseQuery to select
ContactQuery: Rename baseQuery to select
CustomvarQuery: Rename baseQuery to select
DowntimeQuery: Rename baseQuery to select
EventHistoryQuery: Rename baseQuery to select
GroupsummaryQuery: Rename baseQuery to select
HostgroupQuery: Rename baseQuery to select
HoststatusQuery: Rename baseQuery to select