Support for Cassandra NoSQL

Very basic but working support for Cassandra NoSQL 1.2.0. Requires
Cassandra JDBC connector 1.1.2 or later.
Supports:
- creation and using locking and log table
- changesets based on <sql> block with CQL
Fix problem on Liquibase.forceReleaseLocks() for H2 DB

There was a NPE on AbstractJdbcDatabase trying to het columnSize of
LIQUIBASE and MD5SUM
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.
Added support for setting a sequence's current or next value as a default value.
Refactored SequenceFunction into SequenceNextValueFunction and SequenceCurrentValueFunction.
Both now extend DatabaseFunction.
Added method to database to generate the real value for a database function.
Added tests for the new functionality.
Made ColumnMapRowMapper respect SELECT AS column labels
Added support for finding foreign keys on Hsql databases
Merge pull request #101 from stephlag/master

Fix problem on H2 when trying to use forceReleaseLocks
Merge pull request #104 from siebz0r/master

Added support for finding foreign keys on Hsql databases
Checksum Doc and Tests
Documenting and testing ColumnConfig
Documenting and testing ColumnConfig
Added string params for setValueBoolean and setDefaultValueBoolean
ConstraintsConfig Doc and Tests
Documenting DatabaseChange interfaces and removing the isSerializated and Text annotations in favor of better leveraging the existing DatabaseChangeProperty annotation
Make serialization more controllable at an object level rather than containing all the reflection logic within the serializers
Added 3.0 changelog info
getBoolean needs to handle string objects
added some compile locations to gitignore
Merge branch 'sequenceComputed' of https://github.com/vorbis/liquibase into vorbis-sequenceComputed

Conflicts:
	liquibase-core/src/main/java/liquibase/change/ColumnConfig.java
	liquibase-core/src/main/java/liquibase/change/core/AddAutoIncrementChange.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/OracleDatabase.java
Finish merge branch 'sequenceComputed' of https://github.com/vorbis/liquibase into vorbis-sequenceComputed