Add typeForColumn functions

* Add `typeForColumn` and `typeForColumnIndex` methods and tests
* Update warnings in `dataForColumn` and `dataForColumnIndex` to describe SQLite behavior re zero-length blobs
* Bump version to 2.7.8

This should permit accurate handling of zero-length BLOBs, e.g., in Swift:

```swift
let fileURL = URL(fileURLWithPath: NSTemporaryDirectory())
    .appendingPathComponent("test.sqlite")

do {
    let db = FMDatabase(url: fileURL)
    db.open()
    try db.executeUpdate("CREATE TABLE test (data BLOB)", values: nil)
    let data = "foo".data(using: .utf8)!
    let sql = "INSERT INTO test (data) VALUES (?)"
    try db.executeUpdate(sql, values: [data])
    try db.executeUpdate(sql, values: [Data()])
    try db.executeUpdate(sql, values: [NSNull()])

    let rs = try db.executeQuery("SELECT * FROM test", values: nil)
    while rs.next() {
        let type = rs.type(forColumnIndex: 0)
        switch type {
        case .blob:
            if let data = rs.data(forColumnIndex: 0) {
                print("blob with \(data.count) bytes")
            } else {
                print("blob but no data")
            }

        case .null:
            print("Null")

        default:
            print("type is \(type)")
        }
    }
} catch {
    print(error)
}
```

That will produce:

```
blob with 3 bytes
blob but no data
Null
```

1 files changed, 1 insertions, 1 deletions

diff --git a/README.markdown b/README.markdown
index c1fef18..6aa06ed 100644
--- a/README.markdown
+++ b/README.markdown
@@ -78,7 +78,7 @@ Declare FMDB as a package dependency.
 .package(
     name: "FMDB", 
     url: "https://github.com/ccgus/fmdb", 
-    .upToNextMinor(from: "2.7.7")),
+    .upToNextMinor(from: "2.7.8")),
 ```
 
 Use FMDB in target dependencies