Added current date time placeholder function name.

If the function name is matched in UpdateGenerator or InsertGenerator, the value is replaced with the real current datetime.
Using current_datetime as the placeholder.
Updated test xml with an example.
Either valueComputed or valueDate can be used to set the value.

Removed redundant castings in Column.java and unused methods.
Merge branch 'currentDateTimeComputed' of https://github.com/vorbis/liquibase into vorbis-currentDateTimeComputed

Conflicts:
	liquibase-core/src/main/java/liquibase/database/AbstractJdbcDatabase.java
	liquibase-core/src/main/java/liquibase/database/core/DB2Database.java
	liquibase-core/src/main/java/liquibase/database/core/DerbyDatabase.java
	liquibase-core/src/main/java/liquibase/database/core/H2Database.java
	liquibase-core/src/main/java/liquibase/database/core/HsqlDatabase.java
	liquibase-core/src/main/java/liquibase/database/core/InformixDatabase.java
	liquibase-core/src/main/java/liquibase/database/core/MaxDBDatabase.java
	liquibase-core/src/main/java/liquibase/datatype/LiquibaseDataType.java
	liquibase-core/src/main/java/liquibase/sqlgenerator/core/InsertGenerator.java
	liquibase-core/src/main/java/liquibase/sqlgenerator/core/UpdateGenerator.java
	liquibase-core/src/main/java/liquibase/structure/core/Column.java
Finished merge branch 'currentDateTimeComputed' of https://github.com/vorbis/liquibase into vorbis-currentDateTimeComputed