Added fieldtype Bigint

[stephanvd]

We needed fieldtype Bigint, so we added it in. Bigint inherits from Int and is very similar.

[halkyon]

Few things:

• Please remove the whitespace additions, might be your editor automatically doing this?
• Squash the commits together
• Don't use the ?> end PHP tag in BigInt.php, it's not good practise to do that

If we added this, we'd also need to update the other database adapters to support bigint too:

• https://github.com/silverstripe/silverstripe-mssql
• https://github.com/silverstripe/silverstripe-postgresql
• https://github.com/silverstripe-labs/silverstripe-sqlite3

[halkyon]

Technically it's not a 32-bit field, it's an 8-byte (64-bit) field, but I assume this was copied from Int.php. You might also want to mention in the class docs here that PHP running as 32-bit might not work with Bigint properly, as it would convert the value to a float when queried from the database since the value is a 64-bit one.

[simonwelsh]

This will also need pull requests for postgres, sqlite and mssql. Also, should probably be against master versions so that separate pull requests aren't required for those as well.

[stephanvd]

Thanks for the feedback!

I've now done the following:

• Commits squashed together.
• Removed php closing tag.
• Pull requests for postgres, sqlite and mssql are on their way.
• Comment on Bigint changed to reflect reality. Warning added.

A few questions:

• @halkyon Whitespace wasn't added, but unnecessary whitespace was removed. Want me to undo this?
• @simonwelsh Do you mean I should add another request against master? Or replace this one?

[stephanvd]

Related PR's:

• Add bigint support silverstripe-mssql#18
• Add bigint support silverstripe-postgresql#30
• Add bigint support silverstripe-sqlite3#6

[dhensby]

cherry-picked against 3 branch